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TE 

662 

.43 

no. 

FHWA- 

RD- 

81-087 



port No. FHWA/RD-81/087 



ETECTION OF FLAWS IN REINFORCING STEEL 
>N PRESTRESSED CONCRETE BRIDGE MEMBERS 



April 1981 
Final Report 




Document is available to the public through 
the National Technical Information Service, 
Springfield, Virginia 22161 



Prepared for 

FEDERAL HIGHWAY ADMINISTRATION 
Offices of Research & Development 
Structures and Applied Mechanics Division 
Washington, D.C. 20590 



FOREWORD 



This report presents the development of a practical nondestructive 
evaluation (NDE) device to detect loss of section and/or breaks in 
reinforcing steel in prestressed concrete bridge members. In the 
course of the research fifteen NDE methods were assessed. The magnetic 
field disturbance method reported here was selected as having the best 
chance for success in achieving the objective of the study. 

This report is being distributed by memorandum to research engineers 
concerned with the deterioration of prestressed concrete bridge members. 

Research in bridge inspection and evaluation is included in the Federally 
Coordinated Program of Highway Research and Development as Task 2 
"Structural Integrity of Inservice Bridges" of Project 5L, "Safe Life 
Design for Bridges." Mr. Charles H. McGogney is the Task Manager. 



Charles F. Sche'fVey 
Director, Office of Research 
Federal Highway Administration 



NOTICE 

This document is disseminated under the sponsorship of the Department 
of Transportation in the interest of information exchange. The United 
States Government assumes no liability for its contents or use thereof. 
The contents of this report reflect the views of the contractor, who is 
responsible for the accuracy of the data presented herein. The contents 
do not necessarily reflect the official views or policy of the Depart- 
ment of Transportation. This report does not constitute a standard, 
specification, or regulation. 

The United States Government does not endorse products or manufacturers. 
Trade or manufacturers' names appear herein only because they are con- 
sidered essential to the object of this document. 



U2. 



Technical Report Documentation Page 



1. Report No. 

FHWA/RD- 81/08 7 



2. Government Accession No. 



3. Recipient's Catalog No. 



4, Ti tie and Subtitle 



5. Report ~*ate 



DETECTION OF FLAWS IN REINFORCING 
STEEL IN PRESTRESSED CONCRETE 
BRIDGE MEMBERS 



April 1981 



6. Performing Organization Code 



7. Author's) 



F. N. Kusenberger, J. R. Barton 



8. Performing Organization Report No. 

15-4543 



9. Performing Organization Name and Address 

Southwest Research Institute 
Division of Instrumentation 
6220 Culebra Road, P.O. Drawer 28510 
San Antonio, Texas 78284 



10. Work Unit No. (TRAIS) 

35F1-042 



11. Contract or Grant No. 

DOT-FH-1 1-8999 



12. Sponsoring Agency Name and Address 

Office of Research 
Federal Highway Administration 
U.S. Department of Transportation 
Washington, D.C. 20590 



13. Type of Report and Period Covered 

Final Report 

April 1976-February 1979 



14. Sponsoring Agency Code 

S0H36 



15. Supplementary Notes 



FHWA Contract Manager: C. H. McGogney (HRS 



DEPARTMENT OF 
TRANSPORTATION 

FEB fc Q iso^ 



16. Abstract 



,ort is the devel- 



LIBRARY 
The long-range objective of the research s u mma ri z e din th i s 

opment of a practical nondestructive (NDE) method for detecting deterioration in 
the reinforcement of prestressed concrete bridge structural members in situ. A 
detailed definition of the problem is presented and the technical approach is sum- 
marized. The basis for selecting and assessing fifteen NDE methods is reviewed, 
and the results of a limited laboratory investigation of the magnetic method prior 
to developing inspection equipment are summarized. Development of a preliminary 
magnetic inspection equipment is described and many records are presented from 
laboratory evaluations using a 20-ft. (6m) section of Texas Type "C" beam and 
from field evaluations on the Sixth South Street Viaduct at Salt Lake City, Utah. 
Similarities between laboratory and field inspection signatures are indicated; other 
prominent anomalous signatures are shown which correlated with steel elements 
neither known to be present nor shown on the plans; still other field signatures 
are shown which indicated the stirrup configuration in the post-tension girders was 
not in accordance with the plans. Correlation investigations are described which 
illustrate promising electronic signature enhancement and recognition methods for 
discriminating between steel artifacts and deterioration. Recommendations for 
further development are outlined. 



17. Key Words 

Corrosion 
Concrete 
Prestressing 
Nondestructive Testing 



Fracture 
Bridges 
Reinforcing Steel 



18. Distribution Statement 

No restrictions. This document is 
available to the public through the 
National Technical Information Service 
Springfield, Virginia 22161 



19. Security Classif. (of this report) 

Unclassified 



20. Security Classif. (of this page) 

Unclassified 



21. No. of Pages 

197 



22. Price 



Form DOT F 1700.7 (8-72) 



Reproduction of completed page authorized 



TABLE OF CONTENTS 



Page 



LIST OF ILLUSTRATIONS 

I. INTRODUCTION 1 

A. The Overall Problem 1 

B. Detailed Definition of the Problem 4 

C. The Approach 5 

II. SUMMARY OF PRELIMINARY EQUIPMENT 
DEVELOPMENT 1 1 

A. Description of Equipment 11 

B. Laboratory Evaluations 17 

1. Test Specimen 17 

2. Test Procedures and Results 19 

C. Field Evaluation 39 

1. Site Selection and Planning 39 

2. Site Description 42 

3. Inspections and Data Analyses in the Field 42 

4. Correlation Investigations 53 

5. Assessment of Equipment Performance 65 

D. Criteria for Field Use 68 

1. General 68 

2. Procedures for Field Use (Preliminary) 68 

III. SUMMARY OF TECHNICAL BASIS FOR SELECTION 

AND DEVELOPMENT OF MAGNETIC METHOD 71 

A. Design and Development of Preliminary 

Inspection Equipment 71 

B. Other Design Considerations 76 

C. Technical Basis for Selection of Magnetic 

Method 78 



li 



TABLE OF CONTENTS (Cont'd) 



IV. CONCLUSIONS AND RECOMMENDATIONS 81 

A. Conclusions 81 

B. Recommendations 82 

APPENDIX I - Interim Report, March 1977, "Detection of 

Flaws in Reinforcing Steel in Prestressed 

Concrete Bridge Members 
APPENDIX II - FCP Research Review Conferences, Program 

Reviews, and Other Presentations 
APPENDIX III- Personnel Contributing to Overall Program 

Effort 
APPENDIX IV - Field Evaluation Conducted at Manufactured 

Concrete, Inc., San Antonio, Texas 



in 



LIST OF ILLUSTRATIONS 

Figure Title Page 

1 Overall View of Two Fractures from Sixth South 

Viaduct Structure in Salt Lake City 3 

2 Type of Structural Members and Regions Given 

Priority 6 

3 Selected Magnetic Records Obtained During Initial 
Laboratory Evaluation Illustrating Typical Signatures 8 

4 Several Views of Magnetic Inspection Equipment in 

the Field (Salt Lake City, Utah) 10 

5 Overall View of Track Assembly and Inspection Cart 
Installed on Test Beam in the Laboratory 12 

6 Closeup View of Inspection Cart in the Laboratory 13 

7 Closeup View of Inspection Cart 14 

8 View of "Remote" Equipment at Operator's Location 15 

9 Functional Block Diagram for Preliminary Magnetic 
Inspection Equipment 16 

10 Construction Drawing for Texas Type "C" Test Beam 18 

11 Overall View of Test Beam Prior to Setting Forms 20 

12 Closeup View of Steel Configuration Near End "A" 

of Beam 2 1 

13 Closeup View of Steel Configuration Near End "B" 

of Beam 22 

14 Overall View of Completed Test Beam 23 

15 "Flawed" and "Unflawed" Strand and Bar Specimens 24 



IV 



LIST OF ILLUSTRATIONS (Cont'd) 



16 Closeup View Showing Details of Flaw Simulation in 
High-Strength Steel Bars 25 

17 Closeup View of Simulated Flaws in Strands 26 

18 Insertion of Strand in Texas Type "C" Test Beam 28 

19 Insertion of High-Strength Bar, Containing Simulated 
Fracture, in Texas Type "C" Test Beam 29 

20 Sketch of End of Texas Type "C" Test Beam Illustra- 
ting Procedure for Identifying Prestressing Steel 
Configuration Inspected 30 

21 Magnetic Inspection Records Illustrating Signatures 
from Simulated Fractures of 1-3/8 In. (34mm) Bar 
with Several Degress of Fracture End Separation with 

3. 7 In. (9. 4cm) Concrete Cover 32 

22 Magnetic Inspection Records Illustrating Signatures 
from Several Flaw Sizes in 1 In. (2. 5cm) High- 
Strength Steel Bar (Texas Type "C" Test Beam) with 

3. 5 In. (9cm) Concrete Cover 33 

23 Magnetic Inspection Records Illustrating Signatures 
from Several Sizes of Flaws in Strand [0. 5 in. 

(1. 3cm) x 7 wire] (Texas Type "C" Test Beam) 34 

24 Magnetic Inspection Records Illustrating Signatures 
from Flawed Strand [0. 5 in. (1. 3cm) x 7 wire] with 
Several Depths of Concrete Coverage (Texas Type 

"C" Test Beam) 35 

25 Magnetic Inspection Records Illustrating Combined 
Signatures from a Stirrup and Flaw (simulated 
fracture, 0.5 in. separation) as a Function of 
Relative Position (Texas Type "C" Test Beam) for 

1-3/8 in. (3. 5cm) Bar 36 

26 Magnetic Inspection Records Illustrating Signature 
Response from End of a Bar Near the End of Scan 37 



LIST OF ILLUSTRATIONS (Cont'd) 



27 Graph Presenting Signature Response from Several Types 
of Simulated Deterioration as a Function of Concrete 
Coverage 40 

28 Views of Field Test Site at Sixth South Street Viaduct 

in Salt Lake City, Utah 43 

29 Views of Preliminary Magnetic Inspection Equipment 

at Sixth South Street Viaduct, Salt Lake City, Utah 44 

30 Girder Drawings for Sixth South Street Viaduct 45 

31 Post-Tensioning Bar Layout for Sixth South Street 
Viaduct 46 

32 Layout of Sixth South Street Viaduct Spans Showing 
Location of Girders Inspected 47 

33 Magnetic Records from an Inspection Scan Beneath 
Each of the Two Lower Bars on Girder 7 Between 

Bents 27 and 28, Sixth South Street Viaduct 50 

34 Selected Signatures from Sixth South Street Viaduct 
Girders 52 

35 Overall View, After Excavation, in Region of 

Interest on Girder 7, Bents 36-37 54 

36 Closeup View in Region of Signature C, Girder 7, 

Bents 36-37 55 

37 Closeup View, After Excavation, in Region of 

Signature A, Girder 7, Bents 28-29 56 

38 Overall and Closeup Views of Laboratory Mockup 
of Utah Bridge (S. L. C. Sixth South St. ) Girder 

Steel Configuration 57 

39 Closeup Views of Laboratory Mockup of Utah Bridge 
(S. L. C. Sixth South St. ) Girder Steel Configuration 
Showing Stirrups and a Chair 58 



VI 



LIST OF ILLUSTRATIONS (Cont'd) 



40 Selected Inspection Records Illustrating Different 
Stirrup Signatures from Pretensioned Strand and 
Post-Tensioned Bar in Utah Bridge (S.L.C. Sixth 
South St. ) Girders and Corresponding Responses 

from Laboratory Mockups 60 

41 Magnetic Records Showing Corresponding Character- 
istics Between Signatures Obtained from the Field 

and Those from Laboratory Mockups 61 

42 Block Diagram Showing Procedure Used to Subtract 
Magnet Records 63 

43 Computer Reproduced Records Before Subtraction 64 

44 Computer Reproduced Records Illustrating Results 

of Subtraction Process 66 

45 Magnetic Signatures from a Simulated Fracture [0. 5-in. 
(1. 3cm) end separation] of a 1-In. (2. 5cm) Bar 
Illustrating Significant Features of Electromagnet 
Development 72 

46 Magnetic Signature from a Simulated Fracture [0. 5-in. 
(2. 5cm) end separation] of a 1-In. (2. 5cm) Bar for 
Equivalent Concrete Coverage of 9. 5 Inches (24cm) 
(without duct) 73 

47 Typical Magnetic Signature Response as a Function 

of Applied Magnetic Field 75 

48 Magnetic Records Comparing Results Using an 
Electromagnet and Permanent Magnets from Simu- 
lated Fracture in 1-In. (2. 5cm) High-Strength Bar 

in Rigid Duct 77 



VII 



I. INTRODUCTION 

A. The Overall Problem 

About 25 years ago, a new bridge structural design called 
prestressed concrete was introduced. In recent years, the use of pre- 
stressed concrete bridges has been widespread and such design now incor- 
porates a variety of structural configurations. Basically, prestressed 
concrete bridge structural members are of two general types, pretensioned 
and post- tensioned. Current pretensioned construction usually consists of 
7-wire strand, on the order of 1/2-in. (1. 3cm) diameter, arranged in a 
matrix on 2- in. (5cm) centers and the strands are tensioned prior to 
casting the concrete members. Pretensioned members are produced at a 
plant site because of the special fabrication facilities and tooling required. 
In the case of the post-tensioned configuration, ducts, usually metal, are 
cast in a specified location and configuration in the concrete member; sub- 
sequently, the reinforcing strand, rod, or bar is tensioned, usually at the 
bridge site, and grouting material is introduced to fill the space between 
the reinforcement and the duct. 

The load-carrying capability of prestressed bridge structural 
members is directly dependent upon the strength of the steel reinforcement 
rods, bars, or strands; hence, the integrity of this steel is of primary con- 
cern and is influenced by one or more of the following factors: 

(1) Quality of manufactured reinforcement material - 
governed by dimensional tolerances, strength, 
ductility, metallurgical type flaws such as voids or 
impurities, and mechanical damage such as nicks, 
gouges, etc. 

(2) Corrosion deterioration as a result of field environ- 
ment. 

(3) Fracture - failure as a result of over stress (caused 
by loss of section due to corrosion deterioration) or 
by impact loading (as a result of construction or 
vehicular impact). 

(4) Bond between steel and concrete, associated corro- 
sion of post-tensioned members due to voids in duct 
grouting collecting moisture. 



In recent years, there is conclusive evidence that deterioration 
of the steel as a result of corrosion does occur; furthermore, such deterio- 
ration does critically affect the structural strength. ( 1 > 2 , 3) Currently used 
inspection procedures rely heavily on rust staining, cracking, and spalling 
of the concrete as an indicator that a problem exists in the reinforcing 
steel. (2» 4) ^ more detailed discussion of the corrosion deterioration of 
steel in prestressed concrete is presented in Section I. C. of Appendix I ) 
Apparently, deterioration and even fracture of the reinforcement can occur 
without being preceded by visual evidence on the external surfaces of the 
concrete members. For example, the Sixth South Street Viaduct structure 
in Salt Lake City consisting of 192 beams, presently, has more than 21 
bars suspected to be fractured. A photograph of two such fractures is shown 
in Figure 1. In this case, the presence of corroded and fractured post- 
tensioning bars was determined only from i) the loose bars and end-nuts 
during a visual inspection, and ii) the loud noise generated by one of the bars 
breaking which was overheard by people in the area who reported it to the 
State. There are no cracks or significant rust stains visible on the exterior 
surfaces of these particular girders. Spalling of the end-grouting, when bar 
looseness or significant extension is not present, is considered an indication 
of possible bar fracture. 



1. Gould, R. W. , Hummel, R. E. , and Lewis, R. O. , "Electrical 
Resistance as a Measure of Reinforcing Bar Continuity Sunshine 
Skyway Bridge , " Progress Report 1, Dept. of Materials Science 
and Engineering, University of Florida, Gainesville, Florida 32611, 
June 29, 1973. 

2. Lewis, D. A. , and Copenhagen, W. J. , "The Corrosion of Rein- 
forcing Steel in Concrete in Marine Atmospheres," The South 
African Industrial Chemist, Oct. 1957. 

3. Rehm, G. , "Corrosion of Prestressing Steel," General Report 
submitted at FIP Symposium on Steel for Prestressing, Madrid 
1968. 

4. Moore, D. G. , Klodt, D. T. , and Hensen, R. J. , "Protection of 
Steel in Prestressed Concrete Bridges," Research sponsored by 
American Assoc, of State Highway Officials in cooperation with the 
Bureau of Public Roads; Highway Research Board, Div. of 
Engineering, National Research Council, National Academy of 
Sciences - National Academy of Engineering, 1970 



Samples supplied through the 
courtesy of Utah Department 
of Transportation 



1 inch 



2. 54 cm 



FIGURE 1. OVERALL VIEW OF TWO FRACTURES FROM SIXTH 
SOUTH VIADUCT STRUCTURE IN SALT LAKE CITY 



It is evident that presently available inspection methods for 
assessing the condition of reinforcing steel, in situ, in prestressed con- 
crete bridge members are not adequate. The long-range objective of the 
research summarized in this report is the development of a practical non- 
destructive method for detecting deterioration in the reinforcement steel 
of prestressed concrete highway bridge structural members in situ. 

B. Detailed Definition of the Problem 

The above overall objective of the program requires the con- 
sideration of the several flaw categories and an accompanying broad 
spectrum of mechanisms as follows: 

(1) voids - manufacturing and processing flaws 

(2) corrosion deterioration - loss of material by elec- 
trolytic processes, stress corrosion cracking, and 
hydrogen embrittlement 

(3) fracture - overload due to impact, gross loss of sec- 
tion, notch sensitivity, brittle fracture mechanisms such 
as stress corrosion cracking and hydrogen embrittlement 

(4) fatigue 

From an overall point of view, the problem is extremely broad since the 
mechanisms contributing to the decrease or loss of structural integrity 
are complicated and also because of a wide variety of structural designs. 
Therefore, before a logical and valid assessment of nondestructive inspec- 
tion (NDE) methods with possible applicability to the problem could 
effectively be undertaken, it was necessary to more precisely define the 
problem, to detail immediate goals, and to establish priorities according 
to both mechanisms and structural design categories. 

Accordingly, a detailed definition of the problem was under- 
taken based on: 

i) A review of current literature relating to the 

corrosion of steel in reinforced concrete structures. 

ii) Personal contacts with highway and bridge engineers 

in several states where corrosion problems were 
known to exist. 

iii) Conferences with cognizant FHWA personnel. 



A review of the current literature and personal contacts with bridge and 
highway engineers in Utah and Florida indicated that emphasis should be 
placed on the detection of loss-of- section by corrosion or fracture of the 
prestressing steel rods, strands, wires, and bars. In conference with 
cognizant FHWA personnel, the following goals were established for sub- 
sequent efforts under the subject contract: 

(1) Type of Deterioration - primary emphasis on the 
detection of i) 10% or greater loss of area due to 
corrosion of steel, ii) fracture of rod or strand, 
iii) fracture of one or more wires in a strand, iv) 
low priority on the detection of voids, of the order 
of 1/8-in. (3mm) diameter in steel bars (this void 
size is within cross- sectional area tolerance band 
for manufacture of the bars). 

(2) Type of Steel Configurations - both ducted and non- 
ducted steel configurations (pretensioned and post- 
tensioned). 

(3) Type of Structural Members - emphasize the inspection 
of the steel prestressing elements adjacent to and 
essentially parallel to the lower surface of the tension 
flange in "I" and box beams as illustrated in Figure 2. 

It was recognized that, for most applicable nondestructive inspection methods 
a reduction in deterioration detection sensitivity is anticipated for those steel 
elements buried deeper within the concrete beam. 

C. The Approach 

A state-of-the-art literature search and review was conducted 
pertaining to the detection of reinforcing steel deterioration in prestressed 
concrete structures. Based on information contained in the documents 
identified and input from Southwest Research Institute personnel having a 
broad background in related nondestructive inspection problems, fifteen (15) 
NDE methods with possible applicability to the subject inspection problem 
were identified. The formal assessment of these fifteen methods was 
undertaken based upon the detailed definition of the problem (presented in 
the previous section), analysis of information from the published literature, 
NDE background of the assessment team members, and information input 
by the Contract Manager and from other cognizant FHWA personnel. It was 
the concensus of the assessment team that none of the fifteen methods 
offered more than marginal promise for detecting steel deterioration (flaws) 













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Pretension Beam 
(Section, lower region) 



Inspection of region of straight strands 
parallel to and within ~6 inches (15 cm) 
of bottom surface of flange 




Post-tension Beam 
(Section, lower region) 



Inspection of straight bars parallel 
(or nearly parallel to bottom 
surface of flange 




Box Beam 
(Section) 



•Inspection of region of straight strands within 
~6 inches (15cm) of bottom surface 



FIGURE 2. TYPE OF STRUCTURAL MEMBERS AND REGIONS GIVEN 
PRIORITY 

6 



for the case of the reinforcing steel inside a steel duct. A magnetic field 
method was the most promising one, and a limited experimental evaluation 
of the method was conducted. The experimental results were unexpectedly 
good, even with the test bar inside a steel tube; accordingly, it was 
decided to undertake a comprehensive laboratory evaluation of the magnetic 
field method. 

The magnetic field method consists of applying a steady-state 
magnetic field to the beam under inspection and scanning a magnetic field 
sensor along the length of the beam, essentially parallel to each element 
of the prestressing steel, to detect perturbances in the applied field caused 
by anomalies such as deterioration or other types of flaws. A section of 
simulated beam consisting of a wooden superstructure in which a matrix 
of steel bar or strand specimens containing manufactured flaws and un- 
flawed steel elements arranged in various configurations was used for 
laboratory evaluation of the method. The magnetizing field was produced 
by a dc excited electromagnet; a Hall-effect device was used as the mag- 
netic field sensor. Experiments were conducted to determine the influence 
of a range of conditions and test parameters on the detectability of simu- 
lated loss-of-section and fracture. The experiments included varying 
degrees of deterioration, influence of adjacent unflawed steel elements, 
type of duct, type of reinforcing steel, transverse rebar configuration, etc. 
Selected records are shown in Figure 3 which illustrate typical magnetic 
response for several different steel configurations and degrees of deteriora- 
tion. The vertical excursion in each record of Figure 3 is proportional to 
the magnetic disturbance generated at the probe by the steel configuration 
and attendant flaw conditions; generally, there is a large vertical excursion 
at each end of scan (see record C) caused by the demagnetizing effect of the 
specimen ends. The horizontal scale in each record is proportional to 
distance along the specimen, a typical scale of which is shown at the bottom 
of record A. The results of the laboratory evaluation indicated good over- 
all sensitivity to loss-of-section and excellent sensitivity to fracture with 
minimal degradation of signal response in the presence of steel duct. * 



* Detailed results of the laboratory assessment of the magnetic field 

method, as well as the detailed rating and assessment of the many NDE 
methods considered, problem background, discussions related to the 
definition of the problem, etc. , are contained in an Interim Report dated 
March 1977 and entitled "Detection of Flaws in Reinforcing Steel in Pre- 
stressed Concrete Bridge Members," and which is included as Appendix 
I in this Final Report. 




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Based on the very encouraging results obtained in the Labora- 
tory, design and development of a preliminary magnetic field inspection 
equipment was undertaken. Laboratory evaluation work conducted in 
parallel with the development of the preliminary inspection equipment 
using a 20-ft. (6m) section of Texas Type "C" beam produced such 
encouraging results that field evaluations were undertaken Qn the Sixth 
South Street Viaduct in Salt Lake City, Utah, in mid-November 1977 (several 
views of the equipment in the field are presented in Figure 4). Field records 
showed signatures with features similar to those observed for the laboratory 
beam and also several prominent anomalous indications. From analyses 
of records and field excavation at selected locations to establish possible 
signature sources, it was determined that steel elements ("chairs") were 
present in the field structure which were completely unanticipated and 
were not indicated on the construction drawings. Furthermore, subsequent 
simulation of the field configurations and associated signatures in the 
laboratory produced results which confirmed the characteristic signatures 
attributed to the presence of unanticipated elements (chairs) and, in addition, 
showed that the transverse steel configuration in the field girders was not 
in accordance with the construction drawings. Asa result of these findings 
from the limited field evaluations, it has become apparent that an expanded 
program is required to provide solutions for accommodating the variety 
of signature interpretation problems anticipated to be encountered in the 
field. Limited electronic signature enhancement and recognition investi- 
gations conducted in conjunction with the laboratory simulation efforts 
have produced results that show very promising prospects for the develop- 
ment of methods to discriminate between conf igurational steel artifacts and 
the deterioration and fracture of prestressing steel. 

A more detailed presentation and discussion of the field re- 
sults and their interpretation, as well as pre-field laboratory evaluation 
results and a description of the equipment are presented in Section II of 
this report. A summary of the technical basis for the selection and 
development of the magnetic field method is presented in Section III and 
recommendations for continued development of the magnetic field method 
are presented in Section IV. Importantly, if the fifteen candidate NDE 
methods were currently reassessed, the magnetic method would still be 
selected as the best approach for detecting steel deterioration and fracture 
in prestressed concrete members. For nearly all methods other than 
the magnetic approach even more difficult (perhaps insoluble in some 
cases) signature interpretation problems associated with the overall con- 
figuration of steel in field structural members would be anticipated. 




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II. SUMMARY OF PRELIMINARY EQUIPMENT DEVELOPMENT 

A. Description of Equipment 

An overall view of the magnetic field inspection equipment was 
previously presented in Figure 4. For purposes of a more detailed descrip- 
tion of the system, the reader is referred to Figures 5 through 8. Figure 
5 shows another overall view of the equipment with the inspection cart, 
track, and hangers attached to a Texas Type "C" beam (20-ft. test section) 
in the laboratory. The closeup view in Figure 6 illustrates several de- 
tails of the inspection cart, the size of which is commensurate with the 
22-in. width (56cm) of the beam; the hangers are designed such that the 
cart assembly can pass through the hanger, unobstructed, and can scan 
regions of the beam along the entire track length. The total weight of the 
inspection hardware attached to the beam (i. e. , inspection cart, track 
assembly, and hangers) is approximately 460 lbs. (209kg). Figure 7 
illustrates many of the functional elements of the inspection cart. The 
electromagnetic-sensor assembly (which is demountable to facilitate setup 
of the equipment for inspection) may be positioned to different transverse 
locations across and the entire assembly driven along the beam by remote 
control. The capability to use the cantilevered sections of track for 
inspection is significant because it facilitates the inspection of steel 
elements in the vicinity of diaphragm attachment regions and end blocks. 
Figure 8 shows a view of the remote location equipment used by the operator 
during actual inspection; the remote site equipment is shown in the rear of 
a pickup truck for illustrative purposes. A small gasoline engine-driven 
generator (auxiliary equipment) is used to supply the power required by 
the inspection system and all inspection data are recorded on a strip- 
chart recorder (auxiliary equipment). The functional relationship between 
the various system inspection cart and remote site electronic, electrical, 
and control elements is shown by the block diagram in Figure 9. The 
elements which comprise the present inspection system consist of the 
following: 



Approximate Approximate 

Subsystem Size (in) Weight (lb) 

L W H 

Inspection Cart 36 38 8 170 

(less electromagnet/sensor) 

Electromagnet/Sensor 22 8-1/2 12-1/2 91 



11 




FIGURE 5. 



OVERALL VIEW OF TRACK ASSEMBLY AND INSPECTION 
CART INSTALLED ON TEST BEAM IN THE LABORATORY 



12 




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14 




FIGURE 8. VIEW OF "REMOTE" EQUIPMENT AT 
OPERATOR'S LOCATION 



15 



r 



L 



INSPECTION 
CART 1 



Electromagnet 



INSPECTION 
HEAD 



jPower Supply" 



Inspection 

Probe 
Network 







Signal 
Conditioning 



Magnet 
Power 
Supply 



Position 
Indicator 



REMOTE 



Strip 

Chart 

Recorder 



event 

marker 



Transverse 

Position 
Monitor 



Transverse 
Drive 



Limit 
Switches 



Longitudinal 
Scan Drive 



Longitudinal 
Scan 
Control 




Transverse 
Readout 



Transverse 

Drive 

Control 



AC 
Power 
Source 



FIGURE 9. FUNCTIONAL BLOCK DIAGRAM FOR PRELIMINARY 
MAGNETIC INSPECTION EQUIPMENT 



16 



Subsystem 



Approximate 
Size (in) 



W 



H 



23 



10 



19-3/4 13-1/4 



13-1/2 



120ft. 

3-1/2 40 

12-1/2, 3 
15 ft. 



52-1/2 



25 



19 



21 



Approximate 
Weight (lb) 



26 

33 

50 

43, 52 

130 



Signal/Control Rack 

Recorder 

(Auxiliary equipment) 

Signal/Control Cable 

Hangers (each) 

Track Rails (each) 



Portable AC Source 
(Auxiliary equipment) 

1 in. = 2. 54cm 
1 lb. = 0.45kg 
1 ft. = 0. 3m 



The magnetic equipment can be adapted to inspect the lower 
regions on a wide variety of prestressed concrete girders and box beams 
via the track approach. For example, box beams of various widths can be 
accommodated via track attachment brackets (coupled to the box beams by 
inserting anchors) or movable transverse support members. 

B. Laboratory Evaluations 

1. Test Specimen 

Laboratory evaluation of the preliminary magnetic inspec 
tion equipment was conducted using a 20-ft.(6m) section of Texas Type "C" 
prestressed concrete beam. The test beam was fabricated by McDonough 
Brothers, Inc. of San Antonio in conjunction with the production of Type 
"C" beams. To facilitate the later mockup of flawed and unflawed pre- 
tension strands, PVC tubing was placed on a typical 2-in. (5cm) matrix 
spacing; two pretensioned strands were cast in place to achieve adequate 
dead load strength. In addition, metal ducts were cast in place so that 
post-tensioned bar configurations could be simulated using flawed and un- 
flawed bars. Plans for the beam specimen are shown in Figure 10. 
Typical stirrup and "holddown" steel configurations were included in the 



17 




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beam design. Many detailed photographs of the steel "tie-up" were ob- 
tained prior to pouring the test beam; several of these showing the 
internal details of the beam are presented in Figures 11 through 13. 
Figure 14 presents a photograph of the completed beam near the NDE 
laboratory at SwRI; Figure 5 previously presented a view of the beam 
installed inside the laboratory with the preliminary magnetic inspection 
equipment attached. Tests with selected configurations of flawed and un- 
flawed sections of prestressed steel elements were facilitated by the 
fabrication of a set of flawed bars and strands such as those illustrated 
in Figure 15. Simulation of a range of fl w sizes and types of bars was 
accomplished by cutting a 25-ft. (7. 5m) length of bar approximately 7-ft. 
(2. lm) from one end, threading the two mating ends, and generating 
ferromagnetic and nonferromagnetic inserts of different diameters and 
thicknesses; a typical set of such inserts is shown in Figure 16. The 
flaw condition is set up in the bar by selecting the proper insert and using 
a threaded stud, assemblying the two sections of bar with the insert in 
place; for example, a fractured bar with 2- in. (5cm) separation is ob- 
tained by using a 2-in. -long (5cm) brass insert and a length of 300 Series 
stainless steel threaded stud stock (see Figure 16). Flawed strand was 
produced by cutting and removing one or more wires over the desired 
length of the flaw; several examples are shown in the closeup view of 
Figure 17. Table I summarizes the range and type of flaws simulated. 

Pretensioned strand configuration tests were conducted 
by placing one or more flawed strands in a selected position of the 2-in. 
(5cm) x 2-in. (5cm) matrix of PVC tubes and filling the remaining matrix 
with the desired configuration of unflawed strands as illustrated by 
Figure 18. Post-tensioned bar testing was facilitated in a manner similar 
to the strand testing; either 1-in. (2.5cm) diameter of 1-3/8-in. (3.5cm) 
diameter flawed and unflawed bars were placed in one or more of the 
three metal ducts cast in place in the test beam (see Figure 19). 

2. Test Procedures and Results 

Laboratory tests were undertaken utilizing the prelimi- 
nary inspection equipment in a manner similar to that envisioned for 
field testing; the results were recorded on a strip chart recorder. 
Inspections were conducted on various configurations of unflawed and 
flawed strand and bar specimens by inserting the specimens in the Texas 
Type "C" test beam and documenting the flaw location longitudinally along 
the beam via mechanical measurements. The signatures were recorded 
for a number of adjacent scan tracks, magnetizing field strengths (different 
electromagnet excitation currents), scan speeds, etc. Figure 20 illus- 
trates the method used to identify the strand or bar configuration in each 



19 




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Flawed 
Strand 



FIGURE 15. "FLAWED" AND "UNFLAWED" STRAND AND 
BAR SPECIMENS 



24 



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TABLE I. BAR AND STRAND FLAW SPECIMEN 









Loss-of- 


-y- Length 








Type 


Material 


Section (%) 


(in) 




Remarks 


H. 


S. S. # Strand 


1/2 in. 0x7 wire 


14 ; 


0. 12 


1 


wire removed 












14 ; 


2. 


1 


wire removed 












43 ; 


0. 12 


3 


wires removed 












43 ; 


2. 


3 


wires removed 












86 ; 


0. 12 


6 


wires removed 












86 ; 


2. 


6 


wires removed 




i 


t 


^ r 


100 (frac 


ture) 






H. 


S.S. Bar 


1 in. 0, A722 


19 ; 


0. 5 




-- 








Type I, Type II 


19 ! 


2. 




-- 












si ; 


0. 12 




-- 












51 I 


0. 5 
















51 J 


2. 
















ioo ; 


0. 015 




fracture 












ioo ; 


0. 12 




1 












ioo ! 


0. 5 












1 


' 


ioo ! 


2. 




T 








1-3/8 in. 0, A 722 


47 ! 


0. 5 












Type I; Type II 

1 


ioo ; 

ioo ! 
ioo ! 


0. 015 
0. 12 
0. 5 




fracture 

J 




i 


f 


T 


ioo ; 


2. 




♦ 



*H. S. S. - High Strength Steel 
1 in. = 2. 54cm 



27 




FIGURE 18. I.NSERTI OF STRAND IN TEXAS- TYPE "C" 
TES ■ 







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29 



View from End "A" 
(Reference Figure 10 



21 

o 

II 

o 

I 




Legend: 

o Contains no prestressing 

steel 
• Contains unflawed steel 
•jA. Contains flawed steel 



O O • O 

31 32 33 34 

o p o o 

22 23 24 25 

o o o o 

12 13 14 s -' 15 

o o o o 

2 3 4 5 




Transverse 

-« 




Strand or Bar Location 
#21 and #30 pretensioned strands 

are permanently bonded for 

dead load strength 
#33 Strand is not removable 
#14 Duct 1-5/8 in. 0, flexible 
#17 Duct 2-3/8 in. 0, rigid 
Draped Duct 2-3/8 in. 0, rigid 

1 Ln. = 2. 54cm. 



FIGURE 20. 



SKETCH OF END OF TEXAS TYPE "C" TEST BEAM 
ILLUSTRATING PROCEDURE FOR IDENTIFYING PRE 
STRESSING STEEL CONFIGURATION INSPECTED 



30 



inspection. Selected magnetic inspection records are presented in 
Figures 2 1 through 26 which illustrate typical experimental results. A 
brief discussion of each of these figures follows. 

Figure 2 1 presents results which indicate the signature 
response from a 1-3/8-in. (3. 5cm) diameter high- strength steel bar for a 
simulated fracture with varying degrees of separation between the 
fractured ends. The data shown in Figure 21 are for the centerline of the 
flawed bar 3. 7-in. (9. 4cm) beneath the concrete surface of the test beam 
being scanned; the clearance between the magnet pole faces (and Hall- 
effect sensor and the concrete surface of the beam was 0. 5- in (1. 3cm). 
It is pointed out that the signature from the simulated fracture for 0. 5- in. 
(1. 3cm) separation and greater can be readily recognized from those 
produced by the transverse stirrups. Furthermore for the cases in which 
the simulated fracture is located between stirrups, end separations as 
small as 0. 015-in. (0. 4mm) can be discerned because of the large hori- 
zontal extent of the signature. For the record at the top of Figure 2 1, the 
horizontal distance between the upward-going and downward-going peaks 
of both the stirrup and fracture signatures have been indicated. This 
separation between the peaks is proportional to the distance between the 
sensor and the flaw and/or steel configuration causing the signature and 
is a parameter which can be used to identify the depth of the element 
causing the signature. It is noted that the separation of the peaks for the 
stirrups is approximately one-third of that for the simulated fracture; 
correspondingly, the lower arm of the stirrups is approximately 1.5-iri. 
(3. 8cm) from the surface of the girder while the post- tensioned bar is 
approximately 3. 7-in. (9. 4cm) from the surface. This peak separation 
feature of the signatures will be referred to throughout this report since 
it is a parameter which can be extremely helpful in the interpretation of 
inspection results. A comparison of corresponding stirrup signatures in 
the four records of Figure 21 show excellent repeatability even though bars 
were removed and replaced to set up the various flaw conditions. Also, 
it is pointed out that stirrup signatures could be monitored to assess 
possible deterioration in the stirrup regions and to detect missing stirrups. 

Signature responses from varying degrees of reduction 
in cross- sectional area (simulation of loss-of-section due to corrosion)f or a 
1-in. (2. 5cm) diameter high- strength steel bar are shown in Figure 22. 
The upper record (Figure 22) shows the response from a 50-percent re- 
duction in area over a 1/2-in (1. 3cm) length in 1-in. (2. 5cm) diameter 
bar; the insert shows similar response from a slightly lower percentage 
reduction in area for a 1-3/8-in (3. 5cm) diameter bar. In all records of 
Figure 22 for the 1-in. (2. 5cm) diameter bar, the bar centerline is approxi- 
mately 3. 5- in. (9cm) from the face of the concrete beam. Evaluation of 



31 



See Notes 
/ oo« o|o ooo 

(OOOOO ooo 

o o o O o o££)o o o 
oooooiooooo 




Bar Flaw - Simulated Fracture, 2 in. (50mm) 
Separation 




T = 13.5typ- 

Htlillilitn 



H~ Peak Separation 



Bar Flaw - Simulated Fracture, 0. 5 in. (123mm) Separation 




IfjlMiPIliii 
'WO Feet (3m) 



Bar Flaw - Simulated Fracture, 0. 12 in. (3mm) Separation 




Bar Flaw - Simulated Fracture, 0. 015 in. (0.4mm) Separation 




Notes: x Flawed bar, • unflawed bar, T - transverse scan reading 



FIGURE 21. MAGNETIC INSPECTION RECORDS ILLUSTRATING SIGNA- 
TURES FROM SIMULATED FRACTURES OF 1-3/8 IN. (34mm) 
BAR WITH SEVERAL DEGREES OF FRACTURE END 
SEPARATION WITH 3.7 IN. (9.4cm) CONCRETE COVER 

32 



/ 00*0 000 N 

• oooopooo* 
o o o O o c^)o o o 
ooooo'ooooo 



Bar Flaw - 50% Area Reduction x 0. 5 in. 
(12.5mm) Length 




Holdown- 



47% Area Reduction x 0. 5 in. long 
1-3/8 in. 0Bar- ■ ► 



illiililliilMl 

Bar Flaw = 50% Area Reduction x 0. 12 in. (3mm) Length 




Bar Flaw = 20% Area Reduction x 2 in. (5cm) Length 




Bar Flaw = 20% Area Reduction x 0. 5 in. (12. 5mm) Length 




Notes: x Flawed bar, • unflawed bar, T - transverse scan reading 



FIGURE 22. MAGNETIC INSPECTION RECORDS ILLUSTRATING SIGNA- 
TURES FROM SEVERAL FLAW SIZES IN 1 IN. (2. 5cm) 
HIGH STRENGTH STEEL BAR (Texas Type "C" Test Beam) 
WITH 3. 5 IN. (9cm) CONCRETE COVER 

33 



o o • o 



Strand Flaw - 3 Wires Removed Over 2 in. (5cm) 
N Length, First Row 




T = 10. 5 typ- 

u — 



•;i Holdown ■£ 



~10 Feet (3m) 



Strand Flaw = 1 Wire Removed Over 2 in. (5cm) Length, First Row 




Strand Flaw = 1 Wire Removed Over 0. 12 in. (3mm) Length, First Row 



31 




Notes: x Flawed strand, • unflawed strand, T - transverse scan reading 
Strand Matrix 2 in. (5cm) x 2 in. (5cm) typically 
* 1 wire of 7- wire strand removed (14% reduction) 
3 wire of 7-wire strand removed (42% reduction) 



FIGURE 23. MAGNETIC INSPECTION RECORDS ILLUSTRATING SIGNA- 
TURES FROM SEVERAL SIZES OF FLAWS IN STRAND [0. 5 in. 
(1. 3cm) x 7 wire] (Texas Type "C" Test Beam) 



34 



y I N. Strand Flaw - Simulated Fracture Over 2 in. (5cm) 

Separation, First Row 



o • • • H • • • • o 




^10 Feet (3m) 

Strand Flaw = 6 Wires Removed Over 2 in. (5cm) 
^ Length, First Row 

ilrifillf!!'! ifflltilllilMtlllllllfcT 




/ oo»ooooo \ 



Strand Flaw = 6 Wires Removed Over 2 in. (5cm) 
Length, Second Row 




s I \ Strand Flaw = 6 Wires Removed Over 2 in. (5cm) 

' ° • ! X* 2 I • . Length, Third Row 



• • 



o • • O • •O* • ° 




Notes: x Flawed strand, • unflawed strand, T - transverse scan reading 
Strand Matrix 2 in. (5cm) x 2 in. (5cm) typically 



FIGURE 24. MAGNETIC INSPECTION RECORDS ILLUSTRATING SIGNA- 
TURES FROM FLAWED STRAND [0. 5 in. (1. 3cm) x 7 wire] 
WITH SEVERAL DEPTHS OF CONCRETE COVERAGE (Texas 
Type "C" Test Beam) 

35 



• \ Simulated Fracture (0. 5 in. separation, 1. 3cm) 

»oooooooo«| at 13 Ft. Longitudinal Position 



o o • o 

• o o o o 
o o o © o 




Simulated Fracture at 12 Ft. -6 in. Location 

i n}jj*iijj;ipsn|fJHii;jB 




Simulated Fracture at 12 Ft. Location 




Notes: x Flawed bar, • Unflawed bar, T - transverse scan reading 
1 in. = 2. 54cm 



FIGURE 2 5. MAGNETIC INSPECTION RECORDS ILLUSTRATING COMBINED 
SIGNATURES FROM A STIRRUP AND FLAW (simulated fracture, 
0. 5 in. separation) AS A FUNCTION OF RELATIVE POSITION 
(Texas Type "C" Test Beam) FOR 1-3/8 in. (3.5cm) BAR 



36 



Beam End (End A*) 



Beam Mid -Span (End B*) 



Bar End ~10 in. (25cm) 
Beyond End of Scan 

mmm 




End of Scan- 
Bar End- 



Bar End -'18 in. (46cm) 
Beyond End of Scan 




End of Scan— I 



Bar End- 



Bar End «^4 in. (10cm) 
Beyond End of Scan 




m 



mm 



Signature ii 

hi;iiiiih l!l;-s.iiii;;i LiiiiiiidiiiJ 



End of Scan 
Bar End 



Jt 



Bar End ^6 in. (15cm) 
Beyond End of Scan 




End of Scan- 
Bar End- 



Bar End ^2 in. (5cm) 
Before End of Scan 




Bar End **> 6 in. (15cm) 
Before End of Scan 



-4Bar End 
Signature ~T 

' ft 
Bar End ' ' End of Scan 

See Figure 10 for configuration of transverse steel, 




;^-Bar Enc, 
^Signature- 

" tiii±UiL:l.lL:i 

I End of 

Scan 



Bar End- 



FIGURE 26. MAGNETIC INSPECTION RECORDS ILLUSTRATING SIGNA- 
TURE RESPONSE FROM END OF A BAR NEAR THE END 

OF SCAN 

37 



the signatures for the 20-percent area reduction for this concrete coverage 
condition indicates a good probability for the detection of a 10% reduction 
in area provided adequate stirrup signature discrimination could be 
developed. The need for such discrimination against configurational steel 
artifact signatures is discussed in considerable detail later in this 
section of the report. 

Figures 23 and 24 present results from simulated 
flaws in 1/2-in. (1. 3cm) diameter x 7-wire strand arranged in a typical 
2-in. (5cm) by 2-in. (5cm) matrix. Figure 23 presents results for 
varying degrees of strand deterioration with the flawed strand in the 
first row of the matrix (see partial cross- sectional view of the girder at 
the upper left record). Analysis of the lower two records (Figure 23) 
indicates that detection of a 14% reduction in cross-sectional area would 
probably be marginal. The records in Figure 24 show a rapid decrease 
in signature amplitude from the same size flaw for increasing depths of 
the flawed strand in the matrix. The second, third, and fourth records 
from the top of Figure 24 present flaw signature data from an 86% reduction 
in area over a 2-in. (5cm) length [removal of six wires from the 7-wire 
strand over a 2-in. (5cm) length] with this flawed strand 2-in. (5cm), 
4-in. (10cm), and 6-in. (15cm), respectively, beneath the surface; unflawed 
strands adjacent to the flawed strand are present in all cases as indicated 
by the cross-section at the upper left in each record. The results in 
Figure 24 indicate that detection of an 86% x 2 in. (5cm) loss of section 
deeper than the first row of the matrix would probably be marginal. 

The influence of signatures from reinforcement steel 
details, such as stirrups, holdowns, etc. , on flaw signature recognition 
is illustrated by Figure 25. The center and lower records in this figure 
indicate that the field perturbances from the combined steel and flaw 
configurations are essentially cumulative. The combined effects can 
significantly modify the amplitude and shape of the resulting signal; for 
example, note the greater amplitude and significantly reduced horizontal 
separation between the upward-going and downward-going peaks (peak 
separation) of the combined stirrup and flaw signature in the lower record 
of Figure 25 as compared with that for the flaw signature only in the upper 
record of the same figure. Importantly, the outstanding signature from 
the simulated fracture (0. 5-in. separation, 1.3cm), illustrated in 
Figure 25, for a concrete coverage of approximately 3.7-in. (9.4cm) 
indicates that such a condition should be readily detectable. Other ex- 
amples will be presented later in this report, however, from field data 
which indicate that the presence of other types of steel details as well as 
other complicating factors can significantly influence the interpretation of 
inspection data. 



38 



Figure 26 presents numerous magnetic records illus- 
trating detection near the end of scan as related to the detection of 
deterioration (flaw condition) near the end of a girder which is resting 
on a bent (such that an inspection scan to the end of the girder is not pos- 
sible). The results in Figure 26 indicate the presence of a significant 
flaw (such as a bar fracture with 0. 5-in. (1. 3cm) separation can be 
detected even though the scan does not fully extend to the flaw location. 
Based on these results, it is estimated that a significant flaw can be 
detected when it is approximately 6- in. (15cm) beyond the end of scan. 
For example, with a 1 -ft. (0. 3m) footing over the bent, fracture could be 
detected almost within 2-ft. (0.6m) of the end of the girder. If detailed 
inspection near the ends of girders is of critical concern, with special- 
purpose inspection equipment design, closer approach to the girder ends 
may be possible. 

Figure 27 summarizes signature response data as a 
function of concrete coverage for several types of simulated deterioration. 
It is pointed out that the electronic noise level of the equipment is two 
orders of magnitude below the amplitude range of the dominating signatures, 
Importantly, realization of the potential capability of the magnetic inspec- 
tion method is heavily dependent on development of techniques for discrimi- 
nating between signatures from deterioration and those from reinforcement 
steel details. The significance of the discrimination problem will become 
clearer in the subsequent discussion of the field evaluation efforts. 

C. Field Evaluation 



1. Site Selection and Planning 

As a parallel effort to the design, fabrication, and 
laboratory evaluation of the preliminary magnetic inspection equipment, 
site selection and planning for field evaluations were conducted. Through 
discussions with FHWA personnel, personal contacts by SwRI personnel, 
and personal contacts by Mr. A. Leone, Consultant, two candidate sites 
were selected. 

a. One site was the Sunshine Skyway Bridge at Tampa, 

Florida, which was selected on the basis that known corrosion deteriora- 
tion currently exists in that structure. Considerable information about 
the structure was made available through the excellent cooperation of 
Mr. Roger Hove, Director, Office of Bridges, FHWA, Atlanta, Georgia, 
and Mr. Jack Roberts and Mr. Rene Rodriguez, Bridge Maintenance, 
Tampa District, Florida Department of Transportation. 



39 



14 



12 



1/2 in. (1. 3cm) 

x 7 Wire Strand 
i 




£io 



i 
O 



1 

•r-i 
a 6 



d 

W) 
•l-l 



Simulated Fracture in 1 in. 

(2. 5cm)0, 1-3/8 in. (3. 5cm) 

High Strength Bar 



2 in. (5cm) Separation 



0.5 in. (1.3cm) Separation 



0. 015 in. (0. 4mm) Separation 




Amplitude Range 
of Stirrup Signals 
(Test Beam) 



w 

86% x 2 in. (5cm) &$jffl 



Section loss 



>M 



Electronic 

Noise 
(20mV) 

xiJ 



4 6 8 10 

Amount of Concrete Coverage, Inches 
(1 in. = 2. 54cm) 



12 



FIGURE 27. GRAPH PRESENTING SIGNATURE RESPONSE FROM SEVERAL 
TYPES OF SIMULATED DETERIORATION AS A FUNCTION OF 
CONCRETE COVERAGE 

40 



b. The other test site was the Sixth South Street 

Viaduct in Salt Lake City, Utah, which was considered a prime field 
test site since there are known deteriorated members (fractured 
1-3/8-in. (3.5cm) post-tensioned bars) and members which were 
perhaps questionable. Photographs of and construction drawings for 
this structure were made available through the efforts of Mr. A. Leone 
and through the excellent aid and cooperation of Mr. Ray Behling, Chief 
Structural Engineer, Utah Department of Transportation, as well as 
Mr. Robert Frost, Bridge Engineer, FHWA, Utah Division. The 
Sixth South Street Viaduct was selected for the first field trip which 
was conducted during the week of 7-11 November 1977. 



Handling equipment requirements, accessability, 
inspection procedures, etc. at the Utah site were discussed by Messrs. 
C. McGogney, Contract Manager, R. Behling, Chief Structural Engineer, 
Utah DOT, A. Leone, Consultant, and F. Kusenberger, Principal Investigator 
at the FCP meeting in Atlanta on 3-4 October 1977 and in subsequent telecons. 
Also, photographs of the inspection setup (installed on the test beam at SwRI) 
and a brief description of the equipment and installation procedures were 
forwarded to Mr. Behling. The inspection plan consisted of first verifying 
detection of the ends of fractured bars in situ ; the location of bar ends should 
be known since one end of a fractured bar had been removed in each of 
two girders. Subsequently, it was planned to proceed to bars suspected 
to be fractured. 

c. As will become evident to the reader, the Utah 

field evaluation records showed several prominent anomalous signatures 
which subsequent field excavations established to be the result 
of unanticipated steel elements (chairs). Following the Utah tests, labora- 
tory simulation of the field configurations confirmed the features of the 
anomalous signatures. Subsequently, it was decided to conduct a second 
field evaluation on beams having a configuration of steel more nearly 
approximating that of the laboratory test beam - desirably a Texas Type 
"C" girder. Accordingly, it was decided to inspect a group of 80-ft. (24m) 
Type "C" girders available at Manufactured Concrete, Inc. (formerly 
McDonough Brothers, Inc. ) San Antonio, Texas. The inspection sig- 
natures obtained from stirrups and simulated fractures (fabricated in a 
reject girder) on 80-ft. (24m) girders were in agreement with those 
obtained from the laboratory test beam. The "holdown" fixture on the 
80-ft. (24m) girders was different from that incorporated in the 20-ft. 



41 



(6m) test beam and corresponding signature responses were observed. 
Still other small amplitude signatures obtained on the production girders 
(not observed on the test beam) were correlated with "tie wire" scrap 
located on the lower surface of some girders. The results of this most 
recent field work at Manufactured Concrete, Inc. are presented in 
detail in Appendix IV of this report. 

2. Utah Site Description 

Figure 28 shows several views of the Sixth South Street 
Viaduct. It is noted that the area beneath the bridge was used as a con- 
trolled parking area during normal business hours. It was necessary to 
make arrangements to clear parking from those areas required to gain 
access to the girders being inspected. Figure 29 shows typical views of 
the inspection equipment installed on a girder and the remote control 
equipment located at ground level in a station wagon. Figure 30 shows 
plans of the structural details for both post-tensioned and pretensioned 
girder designs used in the structure. Particular attention is called to 
the "L"-shaped stirrups specified for both types of prestressing. It is 
also pointed out that the exterior dimensions of the lower flange region 
of the girders are identical to those of the Texas Type "C" test beam 
used in the laboratory evaluations. Later discussions in this report 
will make reference to the "L" stirrups and associated magnetic signa- 
tures. Details pertaining to the placement of the post-tension bars are 
presented in Figure 31. Magnetic inspections were limited to the two 
lower bars and no attempt was made to inspect the "draped" bars. While 
inspections were limited to those elements previously defined in Section 
I. B of this report, with appropriately designed fixtures, the draped 
elements could also be inspected. Referring to Figure 31, it is pointed 
out that the lower two post-tension bars are not precisely parallel to the 
lower concrete surface of the girder. The distance from the centerline 
of the bar to the lower concrete surface varies from a minimum of 
3. 5-in. (9cm) at mid-span to a maximum of 6-in. (15cm) at the ends 
of the girder. The influence of this "slight drape" on the magnetic sig- 
natures will be pointed out later in this section. The sketch in Figure 3 2 
references the location of the girders inspected. 

3. Inspections and Data Analyses in the Field 

As indicated earlier, a logical inspection sequence 
was planned. Initially, a region known to contain the end of a fractured 
bar was to be inspected to provide a signature from a known field condition 



42 



Overall View of 
Inspection Site 




Installation via 
Hydra ulically 
Actuated 
Scissor-Lift 




Installation via 

Reusable 

Scaffolding 



I 

I 







FIGURE 28. VIEWS OF FIELD TEST SITE AT SIXTH SOUTH STREET VIADUCT 
IN SALT LAKE CITY, UTAH 

43 




Overall View of 
Inspection Cart and 
Track Assembly- 
Installed on Gird 



Trac 



Hanger 



Inspection Cart 



Closeup View of 
Inspection Cart and 
Track Assembly- 
Installed on Girder 




View of Remote 
Site Signal and 
Control Equipment 




FIGURE 29. 



VIEWS OF PRELIMINARY MAGNETIC INSPECTION EQUIPMENT 
AT SIXTH SOUTH STREET VIADUCT, SALT LAKE CITY, UTAH 



44 




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Legend: 

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X Fracture 
Confirmed 
and One End 
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yfc Pretensioned 
Strand 



3 4 5 

Girder 



FIGURE 32. 



LAYOUT OF SIXTH SOUTH STREET VIADUCT SPANS SHOWING 
LOCATION OF GIRDERS INSPECTED 



47 



for comparison with signatures from similar conditions obtained during 
previous laboratory investigations. Such conditions existed in girder 4 
between bents 35 and 36 (Span 35-36) and girder 7 between bents 36 and 
37 (Span 36-37) since one end of the fractured bars had been pulled out 
of the duct in each case (see Figure 32). This plan was not followed; 
Mr. Pete Panos, Bridge Maintenance, Utah DOT, indicated problems 
of gaining access to the girders with the fracture bad ends using 
available equipment (a scaffold /flat-bed truck configuration) because 
of existing curbing in these areas of interest. Mr. Panos suggested 
that initial inspections be conducted on girder 7 of the span between 
bents 27 and 28 of the Sixth South Street Viaduct. This location was 
selected on the basis of a possible bar fracture based on the loss of 
end grouting and a slight projection at the bent 28 end of the lower post- 
tension bar (N>rth bar). It was indicated that the longitudinal location 
of the suspect fracture would probably be near the end of the girder 
opposite that showing the projection (i. e. , bent 27 end). This rationale 
was based on previous observations made when fractured bars were 
physically removed from two other locations in the viaduct. Accor- 
dingly, inspection of girder 7, Span 27-28, was initiated near the 
bent 27 end, but was extended over almost the entire length of the 
girder; transverse locations in the region of both lower bars were 
inspected. Five inspection scans were recorded; approximately 2-in. 
(5cm) outboard of the nominal location of each lower bar, at the nominal 
location of each bar, and midway between the two lower bars. This 
scan procedure was typically followed in all girder inspections. A 
composite scan over approximately 52-ft. (15.8m) of the nominal 
60-ft. (18m) long girder was carried out in three adjacent (end-to-end) 
setups of the track assembly with a slight overlap of longitudinal 
coverage of each scan. 

Next, girder 7 of adjacent Span 28-29 was inspected. 
One of the lower bars (North bar) was suspected as fractured because 
the retaining nut was not seated in the end retaining plate at the bent 28 
end. The inspection was initiated adjacent to the bent 29 end of the 
span and progressed towards the bent 28 end. A very outstanding sig- 
nature was obtained approximately 14. 5-ft. (4.4m) from the end con- 
taining the bar extension (bent 28 end), and a reference mark was 
spray-painted on the beam corresponding to this signature location. 

Finally, inspections were conducted on three other 
girders. On two of the girders (girder 7 of Span 36-37 and girder 4 of 
Span 35-36), one end of each fractured bar had been previously removed. 
The location of the end of the remaining bar in each case was indepen- 
dently measured using "plumber's" and "electricians' s snakes"; sub- 
sequently, signature recordings were obtained in the vicinity of the 



48 



fractured end of each of the bars of these two girders. Several pre- 
tensioned strand girders were also used in the Sixth South Street 
Viaduct and signatures from a typical section were recorded on one 
of these beams (girder 5, Span 39-40). Signatures obtained from the 
four post-tensioned girders inspected will now be discussed briefly 
from the standpoint of analyses made in the field based on information 
known at the time of the field site visit. Following this discussion, 
a retrospective interpretation of the same data will be presented on 
the basis of results from subsequent correlation investigations. This 
"before and after" type of data presentation will better acquaint the 
reader with the overall data interpretation problem. 

Figure 33 shows a composite record over r*j 5 2 -ft. 
(15.8m) of the nominal 60-ft. (18m) long girder 7, Span 27-28 in the 
region of the two lower bars. The numbers above each trace (Figure 33) 
identify outstanding signatures from corresponding longitudinal loca- 
tions for scans of the two adjacent lower bars. The lower amplitude 
signatures in the two scans (see arrows in Figure 33) appear similar 
to those obtained from stirrups in the Texas Type "C" laboratory test 
beam (Figure 21); it is pointed out that the signatures in Figure 33, 
attributed to the stirrup structure, appear inverted to those of Figure 21 
because of the relative direction of travel of the inspection cart. In 
nearly all cases, the outstanding signatures numbered in Figure 33 are 
opposite in polarity to the stirrup signatures (i. e. , from left to right, 
the numbered signatures are upward-going and then downward going 
versus downward-going and then upward-going for the stirrup signatures); 
the peak separation of the numbered and stirrup signatures is similar. 
A review of signature characteristics from simulated fractures in the 
laboratory (refer to Figure 21) in conjunction with examination of the 
recordings from the two lower bars (see Figure 33) of girder 7, Span 27- 
28 of the Sixth South Street Viaduct indicates no evidence of fracture in 
the North bar of girder 7 based on the following rationale: 

i) None of the signatures from the North bar of 

girder 7, Span 27-28 have the combined features of the laboratory sig- 
natures indicated for a bar fracture; namely, the same polarity as the 
stirrup signatures and a peak separation of the order of 6-in. (15cm). 

ii) In nearly all cases, the outstanding signatures 

from girder 7, Span 27-28 appear on both the North bar and South bar 
scans at approximately the same longitudinal locations. The coincident 
occurrence of such similar signatures in two adjacent bars located 6-in. 
(15cm) apart (transversely) cannot be interpreted, logically, as indica- 
tive of fracture in one bar only, and fracture of both bars at the same 
longitudinal location is highly improbable. 



49 



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iii) If any one of the outstanding signatures for the 
North bar of girder 7, Span 27-28 corresponds to fracture, then the 
presence of other signatures having the same features would indicate 
multiple fractures within a single bar; this is highly improbable. 

Signature recordings obtained in the vicinity of the 
fracture of the remaining end of the fractured bar in girder 4, Span 35-36 
and in girder 7, Span 36-37 as well as the one in the region of the outstanding 
signature in girder 7, Span 28-29 are presented in Figure 34. 

Referring to the second and third records from the top 
in Figure 34, the regions from which signatures 1 and 2 were obtained 
coincide with the bar ends in girder 7 of Span 36-37 and girder 4 of Span 
35-36, respectively, as measured from one end of the girder using an 
"electrician's snake" (on girder 7, Span 36-37 measurement confirmation 
was obtained using both plumber's and electrician's snakes). Notice the 
similarities in the two signatures; namely, i) both signatures have a 
polarity opposite to that of the surrounding signals attributed to stirrups, 
ii) both signatures are bipolar, i. e. , downward- and then upward-going, 
and iii) the spacing between the downward- and upward going peaks 
corresponds to approximately 5-6 in. (13- 15cm). It is pointed out that 
laboratory results indicate the signature from the end of the bar should be 
unipolar, i. e. , only upward-going or downward- going; for an example, 
refer to the lower right record in Figure 26. Despite these differences 
between the laboratory and field results, independent physical measure- 
ments of the location of the end of the bar for both girders 7, Span 36-37, 
and girder 4, Span 35-36, confirmed the end of the bar in each case to be 
coincident with the outstanding signatures (signatures 1 and 2, respectively, 
Figure 34). Accordingly, during the field evaluation, the results from these 
two cases associated with the known end of the bar were used as a basis 
to examine the other outstanding signature's A, B and C (Figure 34) which 
are similar in shape and amplitude. A detailed discussion of the results 
presented in Figure 34 was conducted during a meeting on the last day of 
the Utah field evaluation at the State Offices of the Utah Department of 
Transportation. ' ,N From these discussions, it was decided to excavate 
the regions corresponding to the prominent signatures A and C (Figure 34). 
The results of the excavation efforts are discussed next in conjunction with 
the results of subsequent work in the laboratory regarding simulation of 
the steel configuration in the Utah girders. 



* See Appendix II for a list of attendees at this meeting. 

51 



Bent 29 



Norm Bar (14. 0) 



Bent 28 




CCUCHART 



_ Gould Inc., Instrument Systems Division 



Bent 3 5 



North Bar (14.0) 
2- 



Bent 36-*- 




Bent 36 



North Bar (14.0) 



Bent 37- 




Note: Signaturea 1 and 2 coincide with location of known bar enda of two fractured bare. Signaturea A. 
B, and C are auapect fracturee baaed on similarity to Signaturea 1 and 2. 



FIGURE 34. SELECTED SIGNATURES FROM SIXTH SOUTH 
STREET VIADUCT GIRDERS 



52 



4. Correlation Investigations 

As previously indicated, field evaluation of the inspec- 
tion equipment was followed by excavation of one signature location each 
in girder 7, Span 28-29, North bar (see signature A, Figure 34) and in 
girder 7, Span 36-37, North bar (signature C, Figure 34) by Utah DOT. 
An air hammer was used to remove the concrete in the regions of 
interest, and the duct was penetrated to determine the condition of the 
post-tension bar in each case. In neither case was the bar fractured. 
However, a "bar-duct support high-chair" was disclosed at each location 
coincident with the signature of interest (see Figures 35, 36, and 37). A 
"high-chair" is a steel structure, usually several inches (cm) in length and 
height (available in a range of sizes), having four support feet which are 
normally flush with the lower surface of the girder; high-chairs, not shown 
on the bridge drawings or otherwise known to be present, were used, 
apparently, to hold the steel in position and off the bottom of the form 
during pouring of the girders. Correlation between the signatures of 
interest and the high-chairs was excellent. For example, analysis of the 
second and fourth inspection records from the top in Figure 34 indicates 
signature C to be 10 in. (25cm) toward bent 37 from signature 1 (corres- 
ponding to the end of a bar); the measurement in Figure 36 shows the 
chair position to be 10 in. (25cm) from the mark, corresponding to 
signature 1, towards bent 37 (the line painted on the bottom surface of the 
girder corresponding to signature 1 was painted during inspection, before 
excavation). 

A cursory analysis of the characteristic signature anti- 
cipated from the high-chair indicates a shape similar to that observed for 
signatures 1, 2, A, B, and C in Figure 34. Since it is known (from the 
measured distance using a snake) that the end of the bar was coincident 
with signatures 1 and 2 (Figure 34), it is anticipated that the signature from 
the end of the bar is "masked" by the high-chair signature. In order to 
clarify and better define the problems of data interpretation encountered in 
evaluating the Utah field results, laboratory investigations were conducted 
using a mockup steel configuration simulating that of the Utah girder, 
according to the construction drawing, and including other constructional 
artifacts such as high-chairs. Figures 38 and 39 present overall and close- 
up photographs of the laboratory simulation setup. The initial stage of the 
investigation was aimed at confirming the typical signature shapes obtained 
from the stirrup configuration of the Utah girders in the field before adding 
the additional complexity of the high-chair. This initial step was prompted 
because of the similarity in shape of stirrup signatures obtained from the 
Texas Type "C" test beam in the laboratory and those obtained from the 
Utah girder in the field (see Figures 21 and 34); examination of the plans 



53 




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FIGURE 37. CLOSEUP VIEW, AFTER EXCAVATION, IN REGION OF 
SIGNATURE A, GIRDER 7, BENTS 28-29 



56 



Stirrups 



1-3/8-in. 
Bar 



Flexible 
Duct 



Inspection 
Cart 




Longitudinal 
"L" Stirrup 



Transverse 
"L" Stirrup 



Flexible 
Duct 




FIGURE 38. OVERALL AND CLOSEUP VIEWS OF LABORATORY MOCKUP 
OF UTAH BRIDGE (S. L. C. Sixth South St. ) GIRDER STEEL 
CONFIGURATION 



57 



Stirrups 






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FIGURE 39. 



CLOSEUP VIEWS OF LABORATORY MOCKUP OF UTAH 
BRIDGE (S. L. C. Sixth South St. ) GIRDER STEEL 
CONFIGURATION SHOWING STIRRUPS AND A CHAIR 
(without high- strength bar) 



for each of these structural members indicate similar shaped stirrup 
signatures would not be anticipated (refer to Figures 10 and 30 for details 
of the stirrup design). According to the plans, the lower portion of the 
stirrup in the Texas Type "C" beam is "L" shaped similar to that in the 
Utah girder, but the foot of the "L" in the Texas beam is oriented trans- 
verse to the axis of the beam while in the Utah beam it is oriented parallel 
to the axis of the beam. Figure 40 shows typical inspection signatures 
for each stirrup orientation in the laboratory and typical signatures ob- 
tained from the Sixth South Street Viaduct. In the left column of records 
in Figure 40, the stirrup signatures for a scan of one pretensioned strand 
configuration girder (girder 5, Span 39-40) in the Sixth South Street 
Viaduct are presented along with signatures from the laboratory mockup 
of "L" stirrups oriented longitudinally according to the bridge plans - 
a high degree of similarity is evident. The right column of records in 
Figure 40 shows a high degree of similarity between stirrup signatures 
from a laboratory mockup of the "L" stirrups oriented transverse to the 
beam axis and those typically obtained from the post-tensioned bar girder 
configurations in the Sixth South Street Viaduct. Furthermore, a com- 
parison between the left and right columns of records (Figure 40) clearly 
establishes the different magnetic inspection signature response for the 
"Li"-shaped stirrups in the two different orientations. The results of this 
phase of the laboratory simulation efforts strongly indicate that the 
stirrup configuration for the post-tensioned girders in the Sixth South 
Street Viaduct in Salt Lake City, Utah, is not in accordance with the con- 
struction plans which specify stirrups with the leg oriented along the beam; 
the pretensioned stirrup configuration appears to be in agreement with the 
plans. Accordingly, subsequent signature simulation efforts in the labora- 
tory to determine the influence of stirrups and chairs on simulated fracture 
signatures were conducted using the transverse orientation of the "L" 
stirrup. 

Selected inspection records in Figure 41 (low three 
recordings) from laboratory mockup tests illustrate signatures from: 

# a high-chair and a smooth continuous bar 
specimen inside flexible duct, 

# a high-chair which is located coincident with 
the end of a bar, and 

, a high-chair which is located coincident with a 

simulated fracture 1-in. (2. 5cm) separation. 

A reproduction of the bar-end signature region from girder 4, Span 35-36, 
North bar of the Sixth South Street Viaduct (see third record from the top, 
Figure 34) is included at the top of Figure 41 to facilitate comparison. 



59 




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Girder 4, Span 35-36, North Bar 




Laboratory Mockup, "L" Stirrups and Chair 

-(1 



Chair ~ 

rH it 




Laboratory Mockup, "L" Stirrups, Chair, and Bar End 




UyWHl^i^iim^HUHIiW 



Laboratory Mockup, "L" Stirrups, Chair, and Simulated Fracture 

rx i: : : 1 1 . .1. i... . 1.1 i.ir: i.J :...!. ,._,( 

;Chair and Simu- 
lated Fracture 



IHHHi 




a- . ..t 1. 1 ; r 



FIGURE 41. MAGNETIC RECORDS SHOWING CORRESPONDING CHARACTER- 
ISTICS BETWEEN SIGNATURES OBTAINED FROM THE FIELD 
AND THOSE FROM LABORATORY MOCKUPS 



61 



The results presented in Figure 41 show a strong similarity between the 
laboratory mockup signatures and those obtained in the field. Further- 
more, the results indicate that the signature from a fracture (or the end 
of the bar) can be completely "masked" by the presence of a chair signa- 
ture; certainly, visual interpretation of the data with any degree of 
confidence is not possible without the aid of some other type of signature 
analysis or interpretation procedure. The large amplitude of the chair 
signature (which results from the fact that the feet of the chair are very 
close to the inspection magnet/sensor unit), undoubtedly accounts for this 
significant masking effect. The signature from a stirrup can also create 
a masking influence on a flaw signature (for example, see Figure 25) 
although the influence does not appear to be as severe as that from the 
chair (for the configurations investigated). 

Recognizing the severe and complicating influence of 
signatures from steel configurational artifacts on the detection and recog- 
nition of flaw signatures, possibilities for discriminating against signatures 
from such artifacts have been considered. One possibility, that of 
"subtracting out" configurational steel signatures so that signatures from 
deterioration can be recognized, has been investigated on a preliminary 
basis. To facilitate this limited study, the previously described laboratory 
simulation tests included the recording of magnetic inspection signatures 
on analog magnetic tape to facilitate later analog-to-digital conversion of 
the signatures and further processing in the computations laboratory. The 
procedure used is functionally outlined in the block diagram of Figure 42. 
Briefly, the procedure consisted of playback of the analog -signature, 
previously recorded on magnetic tape, through an A-to-D conversion 
system, then storing the digital information in a computer memory. Sub- 
sequently, the stored digital data from two different configurations were 
subtracted from each other, point by point, via a "subtraction program" 
prepared for use on an existing graphics terminal/calculator. The results 
of the subtraction process were then plotted in a format similar to the 
original inspection records and printed out in hard copy form. As an 
example, Figure 43 summarizes the results obtained by applying the above 
described process to records from several different steel configurations. 
The computer reproduced record at the top of Figure 43 shows typical 
signatures obtained with stirrups, a high-chair located intermediate between 
two stirrups, and a smooth continuous bar inside a flexible duct (similar 
to setup previously shown in Figure 41). The center record of Figure 43 
shows the inspection signatures obtained with the same physical configura- 
tion as that used to obtain the upper record, but the chair was removed and 
a simulated bar fracture was added at the same location intermediate 
between the two stirrups. The lower record shows the result of physically 
placing the chair directly beneath the simulated fracture. Referring to 



62 



Field Evaluation System 



Inspection 
Cart 



Control 
and Signal 
Console 



Location 
Markers 



Strip 

Chart 

Recorder 



Inspection 
Signature 



Analog 

Magnetic Tape 

Recorder 



i r 



Sync 



^ Reproduce (at computer facility) 

I 

| Analog 

I Signature 



Digitizing 
System 



Digitized 
Signature 



Intermediate 
Digital 
Storage 



Supervisory 
Computer 



Data 



Graphics 
Terminal 

and 
Calculator 




Hard Copy 
Unit 



FIGURE 42 



BLOCK DIAGRAM SHOWING PROCEDURE USED TO 
SUBTRACT MAGNET RECORDS 



63 



MILLIUOLTS 




Stirrups and Chair 
with Smooth 
Continuous Bar in 
Flexible Duct 



MILLIVOLTS 



MILLIVOLTS 



1206 
1008 
880 
688 
408 
288 

e 

1480 
1288 
1800 
888 
688 
488 
288 



Simulated Bar Fracture 
(1 in. Separation) 




Stirrups with Simulated 
Bar Fracture in 
Flexible Duct 



Chair and 
Simulated 
Bar Fracture 




Stirrups and Chair 
with Simulated 
Bar Fracture in 
Flexible Duct 



1 in. = 2. 54 cm 
1 ft. = 0. 3 m 



FIGURE 43. COMPUTER REPRODUCED RECORDS BEFORE SUB 

TRACTION 

64 



Figure 43 again through point -by -point computer subtraction of the entire 
record at the top from the entire record at the bottom of the figure, the 
result should be a record with a prominent signature caused by the simu- 
lated bar fracture with the influence of the stirrups and chair removed. 
The record at the top of Figure 44 shows the results of such a point-by- 
point subtraction process of the upper and lower records in Figure 43. 
The record previously shown in the center of Figure 43 is repeated at the 
bottom of Figure 44 to permit a visual comparison between the simulated 
bar fracture signature obtained as a result of the subtraction process and 
that obtained from the scan of a physical setup in which the chair was not 
present. (The upward-going peak near the left end of the top record in 
Figure 44 is associated with bar-end effects; the relatively large amplitude 
upward-going spike on the signature attributed to the fracture is a result 
of inaccuracies in the chair signature subtraction setup. ) These results, 
although preliminary, are extremely encouraging in that they provide a 
means for significantly reducing the influence of configurational artifact 
signatures on the overall inspection record so that the potential detection 
capability of the magnetic inspection method can be better realized. 

Based on the insight gained from laboratory simulation 
of the field configurations inspected at the Salt Lake City site, the re- 
corded field data were re-examined. Particular emphasis was placed on 
the re-evaluation of the data recorded from girder 7 of Span 28-29, North 
bar, since this bar was a prime suspect for fracture (because the 
retaining nut was not seated in the end retaining plate at the bent 28 end). 
Re -examination of the record (Figure 33) from this North bar yielded no 
conclusive indications of a fracture, although several other signatures 
were identifiable as suspect for containing fracture signature components; 
however, all such suspect signatures appear to be dominantly controlled 
by the presence of stirrup and/or chair signatures. At this time it is not 
possible, with confidence, to select a specific signature location on the 
North bar of girder 7, Span 28-29, which indicates fracture. It is firmly 
believed, however, that with adequate development of signature analysis 
and interpretation networks and procedures, the point of fracture in this 
bar can be detected and located with the magnetic inspection approach. 

5. Assessment of Equipment Performance 

Based on the limited operating experience obtained in 
the field at the Salt Lake City site, the following observations are made 
regarding the performance of the magnetic inspection equipment. 

An initial checkout of the equipment using a reference 
specimen (refer to Operator's Manual for details) at the field destination 



65 



HILLIUOLTS 



MILLIVOLTS 



1299 r 

1000 - 

800 
680 - 
400 
200 



-200 
1400 

1200 

1000 

309 

600 

400 

200 



Simulated Bar Fracture 
in Flexible Duct with 
Chair and Stirrup Signa- 
tures Subtracted 



(Attributed to inaccura- 
cies in the chair 
subtraction setup) ► 




Attributed to 
Simulated Bar 
Fracture 
( 1 in. Separation) 



Stirrups with Simulated 
Bar Fracture in 
Flexible Duct 



Simulated Bar 
Fracture 
(1 in. Separation) 




FIGURE 44. 



0123456789 

FEET 

1 in. = 2. 54 cm 
1 ft. = 0. 3 m 

COMPUTER REPRODUCED RECORDS ILLUSTRATING RESULTS 
OF SUBTRACTION PROCESS 

66 



confirmed satisfactory system operation. No equipment performance 
problems were encountered as a result of handling and shipping. Equip- 
ment response under field conditions was equivalent to that in the labora- 
tory; excellent repeatability was obtained in both the laboratory and the 
field. Anomalous signatures in the field data established the presence 
of steel elements in bridge girders which were neither shown on the 
plans nor known to be present. Furthermore, characteristic signatures 
in the field data indicated the transverse steel stirrup configuration for 
the post-tensioned girders to be different than that shown on the bridge 
girder plans. 

The selection of access and handling equipment to install 
and set up the inspection equipment on a girder was found to be extremely 
important to achieving an efficient inspection. For example, field 
experience at Utah established that use of a hydraulically actuated scissor- 
lift was significantly more effective than that of scaffolding (refer to photo- 
graphs in Figure 28). With the scaffolding, considerable difficulty was 
encountered in lifting the hangers, track, and inspection cart upon the 
scaffold platform; additionally, the maneuverability and safety of personnel 
on the platform was minimal. Actual use of a hydraulically actuated 
scissor-lift (with a platform'*' 6 -ft (1. 8m) x 12 -ft. (3. 6m) greatly facilitated 
setup, movement, and installation of the inspection system; it is estimated 
that the overall inspection could be speeded up by a factor of two to three 
using scissor-lift type access equipment. The track assembly approach 
for longitudinally scanning girders is functionally adaptable to a variety of 
configurations in the field. 

While the magnetic inspection equipment is basically 
adequate, for field operation, moderate modifications to several operating 
functions would be desirable. Each of these areas is briefly discussed 
below. It was found in the field trials that end-to-end coupling of the track 
assemblies resulted in a faster inspection (particularly when the length 
of the girder is significantly greater than the length of the track). 
Although such track coupling was accomplished via C-clamps and flat 
clamping plates in the field, provisions for a quick-connect/disconnect 
track-to-track coupling would reduce the setup time required to conduct 
an inspection. The preliminary prototype inspection cart incorporates 
provisions for manually selecting one of three inspection longitudinal scan 
speeds. The least speed, 2 in. /s (5cm/s), was included because this 
speed was used during all preliminary laboratory investigations. In order 
to reduce the time required for scanning, two additional higher speeds, 
5 in. /s ( 12. 7cm/s) and 10 in. /s (25.4cm/s), were included. Tests show 
that operationally the equipment is the most functional at a scan speed of 
5 in. /s (12. 7cm/s). Modification of the inspection cart drive speed-changing/ 

67 



coupling system to a single speed unit would reduce inspection cart 
weight by almost 20 lbs (8kg); accordingly, significantly improved equip- 
ment installation characteristics would result. Finally, minor modifications 
to the remote cable system are desirable to further ruggedize the assembly 
and facilitate quicker setup and subsequent movement of the inspection 
system to adjacent locations. 

D. Criteria for Field Use 

1. General 

Only a limited field evaluation of the preliminary NDE 
inspection equipment was conducted; accordingly, only a preliminary 
outline of criteria for field use of this equipment is possible at this time. 
Clearly, the experience gained from the Utah field evaluation and the 
existence of structural steel details in bridge structural members which 
are either not on the construction drawings or do not conform to the con- 
struction drawings indicate that data interpretation in the field may be 
impossible in some cases unless electronic record processing is developed. 
Significant development efforts should be undertaken and completed before 
comprehensive field evaluations are undertaken and such efforts should 
place greatest emphasis on signature analysis and interpretation. Minor 
modifications and improvements to the existing preliminary NDE equipment 
are also desirable and would be cost-effective via improved operational 
efficiency thereby increasing the actual inspection time available for data 
acquisition. More recent field evaluations conducted at Manufactured 
Concrete, Inc. (see Appendix IV) tend to confirm the comments above. 
Additional details regarding further development are presented in 
Section IV - Conclusions and Recommendations, of this report. 

2. Procedures for Field Use (Preliminary) 

Although field use data and experience are as yet ex- 
tremely limited, the following general steps are outlined as significant 
feastures of a viable field use criteria; brief comments are included as 
appropriate. The Operator's Manual as well as this report are identified 
as reference documents. 

a. Inspection Log and Forms 

An inspection log, book, or file should be com- 
pleted for each structure inspected. All notes, comments, diagrams, 
schedules, data logs, etc. pertaining to the inspection should be retained 
in this log or file. Forms should be developed for standard entries to 
ensure a record of all pertinent information. 



68 



b. Pre-Site Visit 

The importance of this tep cannot be over empha- 
sized and the detailed nature of such a visit can vary widely depending 
upon the situation and how well one is acquainted with the structure to be 
inspected. It is essential that the environmental and accessibility features 
of the site be noted, and that handling and access equipment requirements 
as well as the number and type of personnel needed to assist in performing 
the inspection be estimated. As part of this step, consideration should 
also be given to the possibility of minimizing inspection costs by customizing 
the configuration of the track assembly to reduce the overall equipment 
installation time required for setup at each scan location. Suspect or 
critical locations should be identified for particular emphasis during inspec- 
tion. 

c. Establish References 

Maps or diagrams should be prepared to establish 
geographical and structural reference points and directions. It is important 
to establish a methodical approach for identifying the bridge structural 
members and elements, and locations inspected, as well as their relation- 
ship to existing landmarks or bench marks. Existing plans, sketches, or 
diagrams should be used as applicable. 

d. Prepare Data Forms and Data Logging Procedures 

Preparations need to be made in advance for indica- 
ting the regions to be inspected, and the method to be used to identify and 
log the recorded data according to physical location on the structure, date, 
operator, etc. The data form and logging should use nomenclature consis- 
tent with that employed in the member location scheme. 

e. Establish an Inspection Schedule 

This step assures that critical locations receive 
emphasis during inspection and preparation of an inspection schedule ahead 
of time assures that an efficient, effective inspection is conducted at mini- 
mum cost. It also permits each member of the inspection team to be 
informed both from the standpoint of the overall effort as well as specific 
task responsibilities. 

f. Conduct Inspections 

Pre- inspection preparation should include checking 
of the inspection equipment and conducting cleaning and maintenance as 

69 



required. The Operator's Manual should be referred to for operation and 
checkout of the equipment. It is recommended that the equipment be 
operationally checked using the reference specimen (see Operator's 
Manual) at least one time each inspection day. Using a properly prepared 
inspection schedule, as inspections are conducted each item should be 
checked off as it is completed. 

g. Data Interpretation 

As the inspection of each scan setup location is 
completed, signature analysis and interpretation should be completed prior 
to removal of the inspection setup from that location; locations on each 
member being inspected corresponding to suspect signatures should be 
marked in a suitable manner (the corresponding signature should also be 
marked on the inspection record in each case). Marked locations should 
be noted on the inspection diagram so that flaw occurrence patterns, if 
any, can be identified. 

h. Flaw Confirmation 

If applicable or desirable, other techniques or 
methods should be employed to confirm the existence and extent of steel 
deterioration in the regions identified during the magnetic inspection. 
Supplementary methods to be used could be additional critical visual 
inspections and/or removal of concrete cover in the local region of interest. 



70 



in. SUMMARY OF TECHNICAL BASIS FOR SELECTION AND 

DEVELOPMENT OF MAGNETIC METHOD 

A. Design and Development of Preliminary Inspection Equipment 

Upon completion of a laboratory investigation of the magnetic 
field method, which yielded very encouraging results (see Appendix I, 
Interim Report), design and evaluation of a preliminary inspection equip- 
ment was undertaken. From preliminary experimental results using 
electromagnet and permanent magnet approaches for magnetization and 
discussions with the Contract Manager, it was decided to initiate design 
based on an electromagnet having approximately the same size and weight 
as that used during the laboratory assessment phase. (Results of the 
limited permanent magnet investigations are briefly summarized later 
in this section of the report.) Assumed criteria used for design follows: 

i) Total weight of inspection cart - 500 lbs. (230 kg) 

ii) Transverse carriage weight - 300 lbs. (140 kg) 

iii) Maximum grade of beam to be inspected - 5% 

iv) Scanning speed range 2-10-in. /s (50-250mm/s) 

v) Maximum track deflection - 1/8-in. (3mm) 

As a parallel effort to the detailed equipment mechanical and electrical 
design, limited design studies were conducted in an attempt to reduce the 
overall size and weight of the electromagnet required to obtain flaw 
responses at least equivalent to those obtained in the initial laboratory 
assessment phase. As a result of these efforts it was possible to improve 
flaw detection sensitivity while reducing the weight of the electromagnet 
by approximately a factor of 2. Figure 45 summarizes the results obtained 
during the electromagnet development effort, and the significant features 
are the use of large area pole plates and magnetic material in the vicinity 
of the sensor (probe). The potential capability of the magnet method using 
the new electromagnet design is indicated by the significant results in 
Figure 46; the signature from a simulated fracture (0. 5-in. , 1. 3cm 
separation), without duct, for an equivalent concrete coverage of 9. 5-in. 
(24cm) is readily detectable above the electronic noise level of the system. 

An accelerated performance schedule required that the detailed 
equipment design parallel the magnet design studies and as a consequence 
some over design of the inspection carriage and cart components and 
assembly resulted. Nevertheless, the total weight of the inspection cart 



71 



2 :.. 5 -'.?.\.. Prob «- to - S P ccim '' n S P a , ci ^ '" 1 \^°L C 4 X 14.5 *'5-ln. Probe -to- Specimen Spacing 

H ! i ■ ::: ! !!i Probc\ / 



2.ov ty P \.../..:.| !:: .'iF|::j O t- o.sv rr • ; p'H L ■ jvH^-H 7 - 

UjainSOdB) V k T. IV .. .. S ..L LMJ Hf, , , . .," ';,. . fe . .4 ^ V ,V :]'!'. r 





ISO lb. Magnet. Small Area Poles 
0.7 x 7 x 8 Poles 



m 






^Jt 1 ; 75 ^^ 



pi ' 

ifclKI'^t. ^TTrT^rTtrntri*' 25-ttt. Prob «-to.Sp«cim«n Spacing 



180 lb. Magnet. Large Area Poles 

vt:[ : :ii ::l; I ' • ' l ^ -^W-^ 15x3x7 ^i«H¥tir^-H^^ Jl vHi+f^ 



~-jh 



2.7 9 x 12.5 



l.ov . TLL.r ,I_ nj:^ F 1.85V~^J' 



^SA&mkha 



90 lb. Magnet Mockup, Large Area Poles 

^1 : . ::| ::t iT H-''?^V^:n'' I T'l "i'Tl I'°n-Clad Probe tiHirT 



5.0V 



11,1... 



l.OV : ::li : 



...H 2.8V" 

wisaarr 



: .Mt 



90 lb. Magnet Mockup, Large Area Poles, Iron Clad Probe 



Iron-Clad Probe 







'.( Gain 60.dB) t ..f^pi! " .^T^Slfe j 

90 lb. Magnet, Iron Clad Probe, 
New 1st Stage Amplifier (breadboard) 



Narrow Pole Configuration 




1.5 x 3 x7 



0.5V 



:1l i2,7\*| :| : :: gi 




t (Gain 60 dB) | - -— j — ;.-— -j—^.^ :-J ^j;^ | .,[ ^ 2.7 fi 12.5 Leg 

90 lb. Magnet, Iron Clad Probe, 
New 1st Stage Amplifier (breadboard) 



0.5V 



"Xu.iv*:| 



(C ain 60 dB) ^ _ ^. ^ „L..y :il.L.j|.^|. ^ j^ 



IS 

90 lb. Magnet, Iron Clad Probe, 
Narrow Pole Configuration, 
New 1st Stage Amplifier (breadboard) 

* Normalized for a total gain of 80 dB. 

I pound = 2. 2kg 1 inch = 25mm 



9.2 x 7 x 8 Poles i!j* 




90 lb. Magnet, Iron Clad Probe, 

Narrow Pole Configuration, 

New 1st and 2nd Stage Amplifier (breadboard) 



FIGURE 45. MAGNETIC SIGNATURES FROM A SIMULATED FRACTURE (0. 5-in. 
(1. 3cm) end separation) OF A 1-IN. (2. 5cm) BAR ILLUSTRATING 
SIGNIFICANT FEATURES OF ELECTROMAGNET DEVELOPMENT 



72 



10-in. Probe -to -Specimen Spacing 
(no duct) 




0.2V 
(Gain 80. 8 dB) 



90 lb. Magnet, Iron Clad Probe, 
Narrow Pole Configuration, 
New 1st and 2nd Stage Amplifier (breadboard) 



1 pound = 2. 2kg 



1 inch = 25mm 



FIGURE 46. MAGNETIC SIGNATURE FROM A SIMULATED 
FRACTURE (0. 5-in. (2. 5cm) end separation) OF 
A 1 INCH (2. 5cm) BAR FOR EQUIVALENT 
CONCRETE COVERAGE OF 9. 5 INCHES (24cm) 
(without duct) 



73 



is approximately 260 lbs. (118 kg) versus the original design criteria 
value of 500 lbs. (230 kg), a considerable reduction in weight. In field 
applications, weight is always a significant consideration and is a par- 
ticularly important factor in the case of components and units which 
must be handled and installed on the structural element to be inspected. 

Initial laboratory evaluation data were acquired to char- 
acterize response of the equipment for the three manually selectable 
scan speeds and at several electromagnet excitation current levels. 
Signature amplitude data from both transverse stirrups and simulated 
bar fracture (0. 5-in. , 1. 3cm, end separation) were monitored and 
analyzed as a function of scan speed, 2, 5, and 10-in. per second (5, 
12.7, and 25.4-mm per second, respectively). Both the stirrup and 
fracture signature amplitudes were found to be independent of speed, 
as anticipated; essentially all subsequent data were obtained at a scan 
speed of 5-in. per second (12. 7 -mm per second); this medium scan 
speed was selected for general use because the dynamics of scan 
starting and stopping was smoother and a convenient strip chart pre- 
sentation was obtained. 

Inspection data were obtained also from both transverse 
stirrups and simulated fracture (0. 5-in. , 1. 3cm, separation) at three 
different values of excitation current for the electromagnet, 0. 5, 1. 0, 
and 2. Amperes (design current). Figure 47 presents a graph of the 
flaw and stirrup signal amplitudes as a function of magnetizing current, 
and these results indicate the design current of 2 Amperes is adequate 
to provide a strong magnetic field in the steel (1-in. (2. 5cm) bar) 
under inspection for a concrete coverage of 3. 5-in. (9cm); the knee of 
the magnetization curve appears to be at approximately 0.7 Ampere. 

During the initial checkout of the 90 -lb. magnet design, 
the preliminary specimen mock-up configuration was used to evaluate 
the sensitivity capabilities of the magnetic method. The results pre- 
viously presented in Figure 46 indicate that an excellent signal-to- 
electronic noise ratio is obtained from a simulated fracture with 
0. 5-in. (1.3cm) end separation and a specimen-to-magnet/sensor 
spacing as great as 10-in. (25cm) (equivalent to 9. 5-in. of concrete 
coverage). It is pointed out that the record in Figure 46 was obtained 
with a recording sensitivity one order of magnitude (factor of 10) 
greater than that used for recording most of the laboratory evaluation 
data. Furthermore, the data in Figure 46 indicate the electronic 
noise to be on the order of two orders of magnitude less than (1/100) 
typical stirrup signal amplitudes obtained from the Texas Type "C" 
test beam. Importantly, these sensitivity limitation results strongly 
indicate excellent potential for the magnetic inspection method to 
detect deterioration of the prestressing steel in concrete bridge 
structural members. 

74 



i 

o 

■*-> 
i 

aJ 

•r-l 
I— I 

Ph 

a 
< 

i— < 

bO 
•f-i 

W 




0.5 1.0 1.5 

Magnetizing Current, Amperes 



2.0 



FIGURE 47. TYPICAL MAGNETIC SIGNATURE RESPONSE AS A 
FUNCTION OF APPLIED MAGNETIC FIELD 



75 



B. Other Design Considerations 

Early in the preliminary equipment design the feasibility of 
a strand/bar magnetic "tracking" approach was explored. The tracking 
concept envisioned a mechanism based on the magnetic field approach 
which would facilitate automatic transverse positioning of the magnet/ 
sensor unit under a preselected element (strand or bar) as the element 
was longitudinally scanned. Such a device would greatly facilitate the 
future development of equipment to inspect draped strands or bars. It 
was determined that essentially no change in the vertical component of 
the applied magnetic field occurred as a function of the transverse posi- 
tion of the steel element; however, a relatively slow change in the 
horizontal field component as a function of transverse position of the 
steel was observed. In the case of parallel elements (strands or bars) 
the change in signal with transverse position was so broad relative to 
the spacing between the elements (even for spacings as great as 6 -in. 
(15cm)) that the individual elements could not be unambiguously resolved. 
Accordingly, further investigation of the approach was terminated. It 
is suggested that a supplementary magnet/sensor unit having small area 
poles might provide adequate resolution for tracking. * Alternatively, 
automatic tracking might be achieved using optical techniques to follow 
the path of the element "predrawn" on the surface of the concrete bridge 
members. The preliminary inspection equipment incorporates provisions 
for the operator to remotely change the transverse position of the inspec- 
tion head; a digital readout of position is provided. 

Before proceeding with the design of the inspection equip- 
ment based on the use of an electromagnet, the feasibility of using 
permanent magnets was investigated on a limited basis. Use of per- 
manent magnets to develop the magnetic method for field application 
would significantly reduce power requirements and might possibly 
provide significant weight reduction. The investigations were conducted 
using permanent magnets which were readily available; namely, cylin- 
drical (1-in. , 2. 5cm-diameter x 6-in. , 15cm length) Alnico magnets 
and Samarium Cobalt discs (1-in. , 2. 5cm diameter x 3/8-in. , 1cm 
length) magnets. Tests were conducted using the magnets in a number 
of configurations and encouraging results were obtained initially; how- 
ever, after about a 2-week testing period it was determined that a 
significant loss in detection sensitivity occurred. It is anticipated that 
the sensitivity loss occurred as a result of aging effects and demag- 
netization effects of grouping the magnets. Figure 48 illustrates typical 
results using a laboratory electromagnet (from the initial laboratory 
investigations) and essentially the same configuration of permanent 



* A similar approach with small separation between magnet poles might 
also provide adequate resolution of steel artifacts (having small con- 
crete coverage) to facilitate precise longitudinal positioning for 
coincidence - subtraction of artifact signatures. 

76 



180 lb. (82 kg) Electromagnet 
Simulated Fracture (0. 5 in. , 1. 3cm separation) 




Alnico Permanent Magnets (4 each) 
Simulated Fracture (1.0 in. , 2.5cm separation) 



1 




Alnico Permanent Magnets (4 each) 
Simulated Fracture (1. in. , 2. 5cm separation) 




t -tV^^ih i_ ;.. | —f- 






.^M 



~~rtj — t~j~ "i- 't~-T^"'j~'~l j 






Probe-to- specimen spacing 2.5 in. (6.4cm), typical 



FIGURE 48. MAGNETIC RECORDS COMPARING RESULTS USING AN 
ELECTROMAGNET AND PERMANENT MAGNETS FROM 
SIMULATED FRACTURE IN 1 IN. (2. 5cm) HIGH 
STRENGTH BAR IN RIGID DUCT 



77 



magnets over about a 10 -day interval. After reviewing and discussing 
these comparative results between the electromagnet and permanent 
magnet approaches with the Contract Manager, the decision was made 
to continue development of magnetic inspection equipment based on an 
electromagnet design for the following reasons: 

i) lack of repeatable results using permanent magnets, 

ii) long delivery time anticipated for permanent magnets 

of the required configuration, and 

iii) the ability to readily make changes in magnetic field 

strength using an electromagnet. 

Certainly, the use of permanent magnets, as a long-range development 
option, should not be eliminated from consideration; such a development 
effort, if undertaken, however, should include careful consideration of 
the stability of permanent magnets as a function of aging, mechanical 
handling, temperature environment, etc. 

C. Technical Basis for Selection of Magnetic Method 

A brief summary of the technical basis for selecting the 
magnetic field method for further development is presented here. The 
reader is referred to Appendix I for a comprehensive treatment of the 
rating and assessment of 1 5 NDE methods and the subsequent laboratory 
investigation of the magnet field method. 

A state-of-the-art literature search and review pertaining 
to the detection of deterioration in reinforcing steel in prestressed con- 
crete structures were conducted. Both Government and industrial indexes 
were searched using a strategy based on intersecting sub-sets of the 
three descriptors, i) nondestructive testing, ii) concrete, and iii) cor- 
rosion. As a result of this computerized search, selected manual searches, 
and personal communications, 7 2 documents were identified which were 
related to the overall program. Based on information contained in the 
identified documents and input from Southwest Research Institute per- 
sonnel, having a broad background in related nondestructive inspection 
problems, 15 NDE methods with possible applicability to the subject 
inspection problem were identified and are listed below. 

Acoustic Emission Electromagnetic, Nonlinear 

Eddy Current Electromagnetic, Reflection 

Electrical Resistance (Concrete) Electromagnetic, Time-Domain 
Electrical Resistance (Steel) RefLectometry 



78 



Half -Cell Potential Radiography 

Holography Strain -Gage 

Magnetic Field Thermal 

Mossbauer Ultrasonic Scattering 

A formal assessment of these 15 method was undertaken 
based on analysis of information contained in documents from the pub- 
lished literature, the NDE background of the assessment team members, 
the detailed definition of the problem, and information input by the 
Contract Manager and from other cognizant FHWA personnel. 

From the assessment, it was the concensus of the team 
that all 15 methods offered only marginal promise for detecting flaws 
when the reinforcing steel was inside a steel duct. Of the 15 methods, 
the most promising was the magnetic field and at least a limited 
experimental evaluation of this method was warranted. Results were 
unexpectedly good, even with the test bar inside a steel tube; accor- 
dingly, it was decided to undertake a comprehensive evaluation of the 
magnetic field method. 

In the laboratory investigation of the magnetic field method, 
a simulated beam and an existing magnetic circuit, Hall-effect probes, 
amplifiers, power supplies, etc. , were utilized. The simulated beam 
consisted of a wooden super-structure mounted on four nonmetallic 
wheels in which a matrix of steel rod or strand specimens containing 
manufactured flaws and unflawed steel items could be arranged in 
various configurations. The magnetic field was applied via a DC 
current excited electromagnet and the simulated beam was mechanically 
moved past the magnetizing circuit - Hall-effect probe inspection head; 
recordings of the signal output from the Hall-effect probe were made 
using a; strip-chart recorder. The following parameters were explored 
on a preliminary basis: 

(1) Flaw size and configu rational parameters including 
length, section loss, orientation of nonsymmetrical 
flaws and separation of simulated fracture surfaces; 

(2) Influence of adjacent unflawed steel items; 

(3) Scan path with respect to centerline of flawed steel 
items; 

(4) Type of duct; 



79 



(5) Type of reinforcing steel; 

(6) Transverse rebars; 

(7) Probe -to -specimen spacing; 

(8) Magnetizing field strength. 

The investigations were carried out using 5-ft. (1. 5m) 
lengths of 0. 5-in. (12.5mm) strand (7-wire) and 1-in. (25mm) -diameter 
ASTM A7 22 Type I and Type II bars; a range of simulated flaws con- 
sisting of reduced cross -sectional areas of various lengths was obtained 
by machining. 

The following conclusions resulted from this preliminary 
experimental investigation of the magnetic field method, and it was 
recommended that development of an inspection equipment based on 
this method be undertaken. 

(1) Good overall sensitivity to loss-of-section. 

(2) Excellent overall sensitivity to fracture even with 
relatively small end separation (on the order of 
0. 01 -in. , 0.25mm). 

(3) Relatively minimal degradation of signal response 
in the presence of steel duct. 

(4) Presence of reinforcement adjacent to flaw had 
only a slight influence on flaw signal, if adequate 
magnetization was provided. 

(5) Configu rational artifacts, i. e. , helical band on 
the duct, thread-like protrusions on Type II bar, 
etc. , and structural features, i. e. , rebars, bar- 
duct contact, etc. , have relatively minor negative 
type influences on flaw detectability. 

(6) Probe-to-reinforcement spacing, both vertical 

and transverse, is a significant parameter influencing 
overall magnetic response. 

(7) Magnetizing field strength required is a function of 
steel section to be inspected and the distance from 
the magnet and probe to the steel element under 
inspection. 

80 



IV. CONCLUSIONS AND RECOMMENDATIONS 

A. Conclusions 

Both laboratory and field evaluations of the magnetic inspec- 
tion equipment have established the following features and capabilities of 
the method: 

1. Excellent agreement between results in the laboratory 
and the field for similar configurations of reinforcing steel. 

2. Good sensitivity to the details of the configuration of 
steel within approximately 6- in. (15cm) of the girder surface scanned. 

3. Good sensitivity to loss-of-section and fracture in post- 
tensioning steel bars (1- and 1-3/8-in. , 2. 5 and 3. 5-cm diameter) in 
flexible and rigid steel or non-metallic duct for concrete coverages up to 
6-in. (15cm), based on simulated flaws. 

4. Good sensitivity to loss-of-section and fracture in pre- 
tensioning strand (0. 5-in. , 1. 3-cm diameter) for concrete coverage of 2 
to 3-in. (5-7. 5cm), based on simulated flaws. 

5. Excellent signal-to-noise ratio; the electronic noise 
level is two orders of magnitude below the amplitude range of the domi- 
nating signatures. 

6. Signatures from configurational details that are closer 
to the detector (having less concrete coverage) tend to "mask" signatures 
from flaws (loss-of-section and fracture) in steel elements in the same 
region but deeper (farther from the detector). 

7. Significantly reduced "masking" influence of stirrups 
for outermost strands (strands adjacent to transverse vertical faces, or 
sides of beam) in Texas Type "C" beams (stirrups do not extend into this 
region); accordingly, greater sensitivity to loss-of-section and fracture 
in strand is obtained in this region. 

8. Excellent possibilities for improved flaw (deterioration 
and/or fracture) recognition in the presence of reinforcement steel details 
(stirrups, chairs, etc.) based on the results of limited signature enhance- 
ment/recognition investigations (see Appendix IV). 



81 



Realization of the full potential capability of the magnetic method Is dependent 
on development of techniques for discriminating between signatures from 
deterioration and those from a variety of reinforcement details. For 
nearly all other NDE methods even more difficult (perhaps insoluble In some 
cases) signature interpretation problems associated with the overall steel 
configuration in field structural members would be anticipated. In retro- 
spect, if the fifteen candidate NDE methods were currently reassessed, the 
magnetic method would still be selected as the best approach for detecting 
steel deterioration and fracture in prestressed concrete members. 
Accordingly, recommendations for further development and evaluation work 
are made below. 

B. Recommendations 

Results from laboratory and field evaluations strongly indicate 
the need for developing signature enhancement/recognition approaches 
before further field tests are conducted. Magnetic field distribution and/or 
electronic signal analysis approaches appear as very promising solutions 
for the flaw signature discrimination problem. Accordingly, each of these 
approaches is briefly described and discussed below. 

Conceptually, the magnetic field distribution approach for 
discriminating steel deterioration signatures from reinforcement steel 
configurational details consists of the use of an array of magnetic field 
sensors, rather than a single sensor, to scan the structural member and 
to detect the presence of flaws using "pattern recognition" approaches to 
aid in interpreting the magnetic field distribution data obtained. It is 
recommended that preliminary laboratory investigations be conducted 
utilizing an array of sensors. Results would be evaluated on the basis 
of visual analysis of recordings made from selected array combinations 
for various structural steel element configurations (flaws, stirrups, 
chairs, etc. ). 

In addition to sensor arrays, the use of modern digital signal 
processing and analysis techniques offer great promise for the enhance- 
ment and recognition of flaw signatures in the presence of "masking" 
signals from steel reinforcement elements. This signal analysis approach 
consists of scanning a region of a beam, simultaneously digitizing the 
sensor(s) analog output and storing the digital Information in memory. 
Subsequently, the stored digital information is analyzed using one or more 
analysis techniques. Correlation analysis is one very powerful technique 
and is particularly applicable to the problem at hand because the signatures 
from a given configuration of steel for scans over the same region are 
highly repeatable. For example, it is possible to recognize the presence 



of certain signature content (from characteristic conf igurational steel 
elements such as stirrups, chairs, hold-downs, etc. ) in a scan by con- 
ducting auto- and cross-correlation tests on the complete signature 
pattern from that scan. Those regions of the scan record showing a high 
degree of correlation, indicating nearly identical conditions and little or 
no deterioration, could then be eliminated from further analysis; sub- 
sequently, attention could be concentrated on those regions not showing 
high correlation since these would most likely be associated with 
deterioration. The correlation results could also be used to implement 
a "differencing process" by which signatures of no further interest could 
be removed from the scan record via point-by-point subtraction. 
Correlation analyses could also be conducted using functions constructed 
from signatures representative of known combinations of structural 
elements (stirrup and chair, stirrup and flaw, chair and flaw, etc.). In 
any event, the analysis could incorporate provisions to compare the actual 
configuration of steel (via interpretation of magnetic records) with that 
expected (via existing plans). Importantly, it is anticipated that a more 
effective flaw signature enhancement/recognition capability would be 
obtained by carefully combining the sensor array and electronic signal 
analysis approaches. 

Subsequent to the investigation, development, and laboratory 
evaluation of the signature interpretation approaches, it is recommended 
that a field evaluation be conducted again at the Sixth South Street Viaduct 
in Salt Lake City, Utah, to confirm the flaw discrimination capability of the 
signature interpretation approach(es). Based on the previous field 
evaluation conducted in Utah, modifications to the existing inspection 
equipment are also desirable to make the system more readily adaptable 
to the field environment. While, basically, the existing magnetic inspec- 
tion system is adequate for field operation, modification of the drive 
coupling and track rail coupling units would facilitate more effective use 
of the system in the field. Such efforts would be cost effective because 
the significantly improved operational efficiency would increase the actual 
inspection time available for data acquisition. 

Contingent on the degree of success of the magnetic field 
inspection system to assess the flaw problems in the Sixth South Viaduct, it 
is recommended that at least three to four additional one-week field evalua- 
tions be conducted. Preferably, field sites should be selected which differ 
widely in accessibility and environmental problems so that experience can 
be gained for assessing the overall logistics of the inspection system instal- 
lation and operational approaches. Each field evaluation should be preceded 
by a pre- site visit of one to two days to coordinate personnel and handling 
equipment problems and to delineate any special field problem areas. Such 



83 



pre-site visits would be cost effective since the subsequent one-week field 
evaluation effort could be significantly more productive (considering the 
number of program personnel involved as well as the support personnel 
and equipment which must be supplied by the State). 



84 



APPENDIX I 

INTERIM REPORT, MARCH 1977 

DETECTION OF FLAWS IN REINFORCING STEEL-IN 

PRESTRESSED CONCRETE BRIDGE MEMBERS 

(Exhibit) 



TABLE OF CONTENTS 

Page 

LIST OF ILLUSTRATIONS iii 

I. INTRODUCTION 1 

A. Background 1 

B. Definition of the Problem 2 

C. Corrosion of Reinforcing Steel in 

Prestressed Concrete 5 

II. RATING AND ASSESSMENT OF METHODS 9 

III. PRELIMINARY EXPERIMENTAL INVESTIGATION 

OF MAGNETIC METHOD 13 

A. Apparatus 13 

B. Experimental Approach 19 

C. Experimental Results 19 

IV. CONCLUSIONS 42 

V. RECOMMENDATIONS 43 

APPENDIX A - Description of Methods 44 

APPENDIX B - Method Rating Worksheets 65 

APPENDIX C - References 66 



n 



LIST OF ILLUSTRATIONS 



Figure Title Pag< 

1 Overall View of Two Fractures from Sixth South 

Viaduct Structure in Salt Lake City 3 

2 Overall View of Laboratory Setup for Magnetic 

Field Investigations ■'-•' 



Closeup View of Simulated Beam with Reinforcing 
Strand Matrix 

Closeup View of Simulated Beam with Reinforcing 
Steel and Duct Mounted 



8 Closeup View of Strand Specimens with 
Manufactured Flaws 

9 Closeup View of CRS Bar Specimens with 
Manufactured Flaws 

10 Closeup View of ASTM-A722 Type I and Type II 
Bars with Manufactured Flaws 

11 Magnetic Records Illustrating Typical Signatures 
from Duct, Strand, Bar and Rebars 

12 Graph of Flaw Signal Amplitude as a Function of 
Probe-to-Specimen Spacing (3 Amp) 



16 

17 



5 View of Strand Matrix with Transverse Rebars 18 

6 View Showing Electromagnet with Flat Plate Pole Tips 19 

7 Overall View of Typical Test Specimens ^ 



23 



24 



25 



26 



30 



13 Graph of Flaw Signal Amplitude as a Function 

of Magnetizing Current **^ 

14 Graph of Flaw Signal Amplitude as a Function of 

Flaw Length ^ 



ill 



LIST OF ILLUSTRATIONS (Cont'd) 



15 Graph of Flaw Signal Amplitude as a Function 

of Loss of Cross Section 



34 



16 Graph of Flaw Signal Amplitude as a Function 

of Loss of Cross Section 35 



17 Magnetic Records (3 Amp) Illustrating Influence 

of Wire Fracture and End-Separation on Signature 
for Simulated Fracture of 0. 5 In. Q) 7-Wire Strand 



36 



18 Magnetic Records (4 Amp) Showing Influence of End- 
Separation on Signature for Simulated Fracture of 
A722 Type I (Specimen L), 1 In. ()), High Strength 

Bar Centered and Touching in Rigid Duct 37 

19 Magnetic Records (4 Amp) Showing Signatures from 
Simulated Fracture of A722 Type I (Specimen L) 

1 In. (J), High Strength Bar Centered in Rigid Duct 

for Probe-to-Specimen Spacings from 2. 5 In. to 

6. 5 In. With and Without Rebars 39 

20 Magnetic Signatures (3 Amp) for 32-In. Length 
Fractured Bar from Sixth South Viaduct, Salt 

Lake City 40 



IV 



I. INTRODUCTION 

A. Background 

About 25 years ago, a new bridge structural design was introduced, 
that of prestressed concrete. In recent years, the use of prestressed 
concrete bridges has been widespread and such design now incorporates 
a variety of structural configurations. Basically, however, prestressed 
concrete bridge structural members are of two general types, pretensioned 
and post-tensioned. Pretensioned construction usually consists of 7-wire 
strand (on the order of 1/2-in. (1. 3 cm) diameter) arranged in a matrix on 
2-in. (5 cm) centers, the strands being pretensioned prior to casting the 
concrete members. Pretensioned members are produced at a plant site 
because of the special fabrication facilities and tooling required. In the 
case of the post-tensioned configuration, ducts, usually metal, are cast 
in a specified location and configuration in the concrete member; the 
reinforcing strand, rod, or bar is usually tensioned on-site and grouting 
material is introduced to fill the space between the reinforcement and the 
duct. 

The load-carrying capability of prestressed bridge structural 
members is directly dependent upon the strength of the steel reinforcement 
rods, bars, or strands; hence, the integrity of the steel is of primary 
concern and is dependent upon one or more of the following factors: 

(1) Quality of manufactured reinforcement material - governed 
by dimensional tolerances and presence of metallurgical 
type flaws such as voids or impurities. 

(2) Corrosion deterioration as a result of field 
environment. 

(3) Fracture failure - result of over stress (caused by 
loss of section due to corrosion deterioration) or by 
impact loading (as a result of construction or vehicular 
impact. 

In recent years, there is growing evidence that deterioration of the 
steel as a result of corrosion does occur; furthermore, such deterioration 
does critically affect the structural strength. Currently used inspection 
procedures rely heavily on rust staining, cracking, and spalling of the 
concrete as an indicator that a problem exists in the reinforcing steel. 
Apparently, deterioration and even fracture of the reinforcement can 
occur without being preceded by visual evidence on the external surfaces 
of the concrete members. For example, the Sixth South Viaduct structure 



in Salt Lake City consisting of 192 beams, presently, has 21 bars known 
to be fractured. A photograph of two of the fractures is shown in 
Figure 1. In this case, the presence of corroded and fractured post- 
tensioning rods was determined only from i) the loose rods and end-nuts 
during a visual inspection, and ii) the loud noise generated by one of the 
rods breaking which was overheard by people in the area who reported 
it to the State. There are no cracks or significant rust stains visible on 
these particular girders. 

It is evident that presently available inspection methods for 
assessing the condition of reinforcing steel, in situ, in prestressed con- 
crete bridge members are not adequate. The objective of the subject 
program is to develop a practical nondestructive method for detecting 
flaws in the reinforcement of prestressed concrete highway bridge members. 

B. Definition of the Problem 



The overall objective of the program as stated in the previous 
section of this report involves the consideration of several flaw categories 
and an accompanying broad spectrum of mechanisms: 

i) voids - manufacturing flaw 

ii) corrosion deterioration - loss of material by electrolytic 

processes, stress corrosion cracking, and hydrogen 
embrittlement 

iii) fracture - overload due to impact, gross loss of section, 
notch sensitivity, brittle fracture mechanisms such as 
stress corrosion cracking and hydrogen embrittlement. 

Certainly from an overall point of view, the problem at hand is extremely 
broad both from the standpoint of mechanisms and also because of a wide 
variety of structural designs. Before the first significant step, that of 
assessing nondestructive inspection (NDE) methods with possible applica- 
bility to the problem ,could effectively be undertaken, it became apparent 
that a more precise definition of the problem was required. Furthermore, 
the definition must include detailed immediate goals accompanied by a 
set of priorities relating to both mechanisms and structural design 
categories. In order to achieve adequate problem d efinition, the following 
steps were taken: 

(1) A review of current literature was conducted relating to 
the corrosion of steel in reinforced concrete structures. 




Samples supplied through the 
courtesy of Utah Department 
of Transportation 



1 inch 



2. 54 cm 



FIGURE 1. OVERALL VIEW OF TWO FRACTURES FROM SIXTH 
SOUTH VIADUCT STRUCTURE IN SALT LAKE CITY 



(2) Personal contacts were made with highway and bridge 
engineers in several states where corrosion problems 
were known to exist. 

(3) Conferences were held with cognizant FHWA personnel. 

The paragraphs which follow in this section summarize definition of the 
problem. 

Review of the current literature, which is briefly summarized in 
the section that follows, showed that the bulk of investigative work con- 
cerning the corrosion of reinforcing steel was aimed predominantly at 
material loss in the reinforcement (accompanied by cracking and spalling 
of the concrete) with much less emphasis on stress corrosion cracking 
and/or hydrogen embrittlement mechanisms. The literature, however, 
did reflect an increasing concern about the possible role of stress 
corrosion and hydrogen embrittlement mechanisms with the wider use of 
high strength steels. Based on the literature review and personal contacts 
with bridge and highway engineers in Utah and Florida, it was decided that 
emphasis should be placed on the selection, rating, and investigation of NDE 
methods for detecting substantial loss of steel section by corrosion and 
for detecting fracture of reinforcing rods, strands, wires, and bars. This 
problem definition was agreed on in conferences with cognizant FHWA 
personnel and included the following: 

(1) Primary sensitivity goals are the detection of i) 10% or 
greater loss of area due to corrosion of steel, ii) fracture 
of rod or strand, iii) fracture of one or more wires in 

a strand. 

(2) Both ducted and non-ducted steel configurations (pre- 
tensioned and post-tensioned) are to be considered. 

(3) Regarding structural configurations, priority is to be 
given to the lower row of steel components along the 
tension flange in "I" and box beams. 

(4) Low priority is to be given to the detection of voids, of 
the order of 1/8-in. (3 mm) diameter in steel bars (this 
void size is within cros s- sectional area tolerance band 
for manufacture of the bars). 

As a result of several interacting factors associated with problem 
definition and establishment of meaningful and realistic deterioration detection 
goals, the program is presently about 9 to 10 months behind schedule but is 
on a well directed course with goals related to currently known field problems. 



C. Corrosion of Reinforcing Steel in Prestressed Concrete 

In view of the fact that corrosion is such a dominant factor for the 
deterioration of steel in a field environment, it will be of benefit to the 
reader to briefly summarize here pertinent information gleaned from 
the literature review. 

Background 

With few exceptions, concrete, because of its alkalinity, forms a 
protective environment for reinforcing steel/ ' ' ' ' The chem- 

ically protective nature of the concrete may be only an initial condition, 
however, and careful consideration must be given to what takes place once 
a structure has been completed and put into service. Steel in prestressed 
bridge structural members is vulnerable to corrosion for several reasons: 

(1) Concrete is permeable^ ' ' ; and even in the absence of 
cracks or spalls reinforcing steel can undergo corrosion from de-icing 
salts, salt water, salt water atmospheres, and/or air pollutants in the 
vicinity of industrial complexes permeating the concrete cover over the 
steel. 

(2) Cracking or loss of concrete cover over the reinforcing steel 
for whatever reason (overload cracks, freeze-thaw cracks, cracks due 
to design difficulties, vehicular impacts, etc. ), provides an opening for 
corrosive atmospheres directly to the reinforcing steel. 

(3) On prestressed structural elements, especially post- 
tensioned ones, the ends of the structure are particularly vulnerable to 
corrosion because of the inherent configuration. The susceptibility of the 
ends of girders or beams in the post-tension case is greater because of 
the necessity to grout the ends after the tensioning has been completed. 

In addition, if the region between the reinforcing steel and the surrounding 
duct is not adequately grouted, the entire length of the reinforcing steel 
becomes susceptible to corrosion. 

Nature of Corrosion 

Generally, the corrosion of steel in concrete appears to be electro- 
lytic or galvanic in nature.^- 5 » 4 ^ ' ' ' Since there are many excellent 



Superscript numbers in parentheses refer to References in Appendix C. 



treatices on galvanic corrosion, the subject will not be treated in depth 
in this report. It is important, however, to recognize that the dominate 
features of this process are the formation of anodic and cathodic regions 
in the presence of an electrolyte (in the case of bridges this is usually 
a chloride), with the loss of metal occurring at the anode where the 
steel goes into solution. Usually, the anodic region is small compared 
to the cathodic region and the loss of material is in the form of severe 
pitting. ' A number of investigators have stated that the corrosion 

process requires not only the presence of an electrolyte but also of 
oxygen; this factor places importance upon the texture of the concrete 
as well as the occurrence of pore space or voids in the vicinity of the 
steel.* 46 ' 25 ) 

Investigations have been conducted which cover a wide range of 
factors influencing the effect of corrosion on the strength of steel.' '' 
45,49,28) The influence of a corrosive environment on the structural 
strength of steel has several facets, and there is some evidence ' 4t) ' 
that prestressing the steel in tension may accelerate the rate for intensity 
of metal removal by corrosion. While electrochemical action results in 
a reduction in cross- sectional area of the steel, what is more significant 
is the notch effect produced by the pit nature of the corrosion. ( 4b ) 
Importantly, the notch effect not only influences the ultimate strength but 
also the ductility of the steel. ( 4fc) ' 

While it is not intended to discuss them here, other possible 
corrosion related mechanisms of failure which could be present are stress 
corrosion and hydrogen embrittlement. However, these brittle fracture 
mechanisms do not appear to be worthy of concern in prestressed bridge 
structures at this time. A number of stress corrosion and hydrogen 
embrittlement laboratory investigations have been conducted on bridge 
steels (25 , Zo, 4b, 5U). th us far, the results appear to be contradictory. 

Some Results of the Corrosion of Steel in Concrete 

It has been widely accepted that the corrosion products occupy 
slightly more than twice the volume of the steel present prior to corro- 
sive attack which can produce pressures of the order of 5,000 psi 
(3. 5 x 10 Pa) in the concrete. (25,4/') This volumetric expansion of 
corroding steel results in the cracking and spalling of concrete 
surrounding the reinforcing steel. The degree of corrosion necessary to 
cause cracking and spalling of the concrete appears to vary widely; e. g. , 
in the laboratory it has been shown to vary from 1 to 30 mils (0. 025 to 
0.75 mm) of material loss necessary to cause cracking.^ 4 "' It seems 
that in most instances, cracking of the concrete caused by corrosion of 
the steel is accompanied by visible signs of rust staining on the exterior 



surface of the concrete but instances have been reported of the formation 

■p C AC) 

of a brown stain prior to cracking. '*-'' ^■ J > 

Numerous laboratory and field investigations of the corrosion 
phenomena have been conducted using the half-cell potential method. This 
method can distinguish between passive and active states of corrosion 
which has provided some fundamental insight into the corrosion process. 
For example, investigations (^o, b 1) h ave reported that under some circum- 
stances the half- cell potential alternates between passive and active values. 
There appears to be consistency, however, in that the potential changes 
from passive to active and remains active sometime (more than several 
weeks) before cracking of the concrete around the steel occurs. Other 
investigations involving multiple wire strands (^9) have shown a tendency 
for more severe corrosion of the internal center strand than for the 
surrounding strands. To summarize the reported findings, there appears 
to be general agreement with regard to the following: 

(1) It is not necessary to have cracking or spalling of the concrete 
in order to have corrosion of the steel take place. 

(2) It appears that at some point during the corrosion process of 
the steel, cracking of the surrounding concrete will take place which will 
eventually be accompanied by spalling of the concrete. 

(3) The depth of corrosion necessary to initiate cracking and/or 
spalling varies widely. 

(4) Concrete cracking, due to corrosion of the steel , is accompanied 
by (or may be preceded by) the appearance of rust stains on the exterior 
surface of the concrete in the vicinity of the steel. 

Observations in the field '^ ' / have noted cracking of the concrete in 
line with some rebars (and ducts) wherein corrosion in the stirrup section 
has appeared to lead to corrosion of the rebars. This perhaps emphasizes 
the importance of adequate concrete coverage over stirrup sections as well 
as the main structural steel members. It was also noted in on-site obser- 
vations that the concrete cracking along the rebar ducts near the middle of 
the beam was not accompanied by rust staining. 

Results reported, although scattered, indicate that the rate of 
corrosion can vary widely. In the laboratory, rates from a mean rate of 
35 mils/yr (0. 88 mm/yr) to a maximum of 138 mils/yr (3. 45 mm/yr) have 
been reported^ 4 "' In a California bridge less than 10 years old^- 3 ^', a mean 
rate of metal loss of 29 mils/yr (0. 725 mm/yr) (range of 7 to 80 mils/yr 



or 0. 18 to 2. mm/yr for a 10 to 90% frequency) was observed. These 
results were noted for the No. 4 reinforcing steel in the bridge deck. 
Another investigator ^°^ estimates a 6 to 10 mils/yr (0. 15 to 0.25 mm/yr) 
metal loss rate under field conditions which he compares to a 7 to 35 
mils/yr (0. 18 to 0. 88 mm/yr) rate for a bridge in England without 
concrete cover. To establish some perspective, based on an average 
metal loss rate of 30 mils/yr (0. 75 mm/yr) from corrosion, a 1/2-in. 
(1. 3 cm) diameter rebar would lose 10% of its original cross- sectional 
area in about 10 months. 

Studies have also been conducted to determine the effect of crack 
width in the concrete on the rapidity of corrosion of the steel. It is 
important here to recognize that this relates to the influence of cracking 
in concrete and the corrosion of steel where the initial crack formation 
is not a result of the corrosion process. The studies have shown that 
cracks as narrow as 0. 004 inch (0. 1 mm) result in rapid corrosion of 
the steel. ^^> Most investigators indicate that the corrosion rate of 
steel once it is unprotected by the concrete (such as in the presence of 
cracks and spalling) would be significantly greater. 



II. RATING AND ASSESSMENT OF METHODS 

A state-of-the-art literature search and review related to the 
detection of deterioration of reinforcing steel in prestressed concrete 
structures were conducted. Both Government and industrial indexes 
were searched using a strategy based on intersecting sub-sets of 
descriptors pertaining to i) nondestructive testing, ii) concrete, and 
iii) corrosion. As a result of this computerized search, other selected 
manual searches, and personal contacts, 72 documents have been identified 
which are related to the overall problem. Numerous other documents 
which provide background information related to applicable nondestructive 
inspection methods also have been identified for use during the method 
assessment task. 

Based on information contained in the identified documents and input 
from Southwest Research Institute personnel having a broad background in 
related nondestructive inspection problems, 15 NDE methods with possible 
applicability to the subject inspection problem were identified. The assess- 
ment of these 15 methods was undertaken based on analysis of information 
contained in documents from published literature, the NDE background of 
the assessment team members, information input by the Contract Manager 
as well as other cognizant FHWA personnel. As pointed out in the intro- 
duction of this report, specific program objectives, NDE method sensitivity 
goals, and inspection priorities (regarding structural configurations) were 
established and agreed upon through suggestions, comments, and meetings 
with FHWA personnel. The establishment of specific objectives and goals 
was crucial to the effective and meaningful assessment of the 15 NDE 
methods which follow: 

Acoustic Emission Holography 

Eddy Current Magnetic Field 

Electrical Resistance (Concrete) Mossbauer 

Electrical Resistance (Steel) Radiography 

Electromagnetic, Nonlinear Strain-Gage 

Electromagnetic, Reflection Thermal 

Electromagnetic, Time-Domain Ultrasonic Scattering 

Ref lee tome try 
Half- Cell Potential 

All documents identified during the search phase of the assessment 
effort were given a cursory review and each categorized according to the 
inspection method(s) discussed in each document; those documents pro- 
viding overall background to the problem were also recognized. Each 
method was rated by a member of the assessment team using a worksheet(s) 



and accompanying instructions (See Appendix B for worksheet format, 
instructions, and completed worksheets for each method assessed). 
Briefly, the worksheet provided a rating of each method based on the 
following parameters: 

(1) Sensitivity, no duct or nonmetallic duct; 

(2) Sensitivity, metallic duct; 

(3) Adaptability to field use; 

(4) Instrumentation factors. 

From the detailed information contained in the completed worksheets, a 
composite comparative rating chart (see Table I) was constructed based 
on the following parameters: 

(1) Sensitivity, no duct or nonmetallic duct; 

(2) Sensitivity, metallic duct; 

(3) Ability to accommodate field environment; 

(4) Ability to access inspection areas; 

(5) Potential for development within scope for the contract. 

A discussion of the composite rating chart (Table I) is presented 
here; for a more detailed examination of ratings for a particular method, 
along wit;h critical comments, the reader is referred to Appendix B. The 
composite rating in Table I utilizes graduations of color (shades of gray) 
to illustrate a rating code because it facilitates a more effective overall 
assessment and comparison of the individual methods. In general, the 
darker the shade (the greater the density) the better the performance of 
the method for that particular parameter; a legend is presented in the 
chart which annotates the meanings of the density rating. Briefly, the 
composite rating chart indicates the following: 

Without Duct or With Nonmetallic Duct 



Magnetic field and radiography were the only promising methods 
with any significant quantitative capability for corrosion detection, 
or partial or total fracture detection. Electromagnetic non- 
linear and reflection methods as well as ultrasonic scattering 
methods appear to offer some possibilities, but would require 



10 



considerable laboratory effort initially to estimate feasibility. 

In addition, the electromagnetic nonlinear method does not offer much 
promise of being quantitative. While the methods commented on thus far 
do not include those requiring penetration of the concrete, even if pene- 
tration of the concrete were considered as a viable option, the added 
methods for serious consideration, for example electrical resistance, do 
not offer significant promise in terms of detection sensitivity. 

With Metal Duct 

Only the magnetic field method offers significant promise; if pene- 
tration of the concrete is considered operationally unacceptable, 
the remaining methods have little capability. As in the no-duct 
case, even if penetration of the concrete is considered a viable 
approach, the capability of methods added for consideration is 
rather poor. 

For the reader wishing to gain somewhat greater insight into the applica- 
bility of each of the methods to the inspection problem at hand, it is 
recommended that the "remarks" in the worksheets of Appendix B for each 
of the methods be perused and that the description of methods in Appendix A 
be reviewed. 

Based upon the results of the literature study assessment of the 15 
methods, the decision was made to conduct a limited experimental evalua- 
tion of the magnetic field method. The objective of this experimental 
effort was to define in somewhat greater detail the capability of the magnetic 
field method, including an initial appraisal of the influence of factors, such 
as metallic duct, stirrup rebars, multiple strand or bar matrix configura- 
tion, etc. on flaw detectability. The experimental work conducted is 
described and the results obtained are discussed in the section which follows, 



11 



GLOSSARY OF TERMS IN TABLE I 

1.0 Methods: Self-explanatory 

1.1 No.: Self-explanatory 

1.2 Description: Self-explanatory 

2.0 Sensitivity: Pertains to the effectiveness of a method to determine the condition of reinforcing steel 
in prestressed concrete. 

2.1 No Duct or Nonmetallic Duct: Rating relates to pre-tensioned configuration for no duct or sheath 
over the reinforcing steel or a nonmetallic duct post-tensioned configuration. 

2.1.1 Corrosion: Relates to the detection of the loss of section due to corrosion. Sensitivity for various 
thickness of concrete coverage categories indicates influence of coverage on detection. 

2.1.2 Fracture: Relates to the detection of the fracture of reinforcement rod or strand. Fracture may be 
a result of overload because of reduced section due to corrosion or the result of mechanical damage from 
impact. 

2.1 .2.1 Total: Detection of fracture of total strand, or rod, or bar. 

2.1 .2.2 2-4 Wires: Detection of fracture of 2 to 4 wires in a strand. 

2.1.2.3 1 Wire: Detection of fracture of 1 wire in a strand. 

2.1.3 Voids: Detection of voids 1/8-inch (3 mm) diameter or larger occurring in bar during fabrication. 

2.2 Metallic Duct: See 2.1 except reinforcement rod, bar, or strand is within a metallic duct (usually 
steel). Rating generally relates to post-tensioned configuration. 

2.2.1 See 2.1.1 

2.2.2 See 2.1.2 

2.2.2.1 See 2.1.2.1 

2.2.2.2 See 2.1.2.2 

2.2.2.3 See 2.1.2.3 

3.0 Accommodate Field Environment: Rating indicates ability of each method to accommodate to 

operation under field conditions. 

4.0 Access Inspection Area: This rating indicates the capability of each method to assess anticipated 

bridge inspection regions because of size, shape or weight of instrumentation package or sensors. Requirement 
to have opposing sides accessible or penetrate concrete (and duct) is indicated by note. 

5.0 Potential for Development (within scope of contract): Rating indicates probability that the method 

can be developed within the scope and funding of the present contract. The scope includes a breadboard 
system, evaluation of the breadboard in the laboratory and the field, breadboard system update (or fabricate 
preliminary prototype system), and field use criteria development. 



12 




13 



III. PRELIMINARY EXPERIMENTAL INVESTIGATION 
OF MAGNETIC METHOD 

A. Apparatus 

The limited experimental effort conducted consisted of a preliminary 
laboratory investigation of the magnetic field method using a simulated beam 
and existing magnetizing circuits, Hall-effect probes, amplifiers, power 
supplies, etc. A test bed was designed in conjunction with the simulated 
beam specimen to permit utilization of an existing mechanical drive system 
to provide scanning motion for the experiments conducted. The laboratory 
test setup is illustrated in Figure 2, along with annotation of the major 
system components. The test bed consists of a wooden frame over which 
the simulated beam can be moved on wheels for mechanical scanning of 
the beam specimen. The simulated beam is a wooden structure mounted 
on four nonmetallic wheels; nonferromagnetic fasteners and hardware were 
used throughout to eliminate possible interference with the magnetic data 
from flaws. 

The simulated beam structure was designed such that the configura- 
tion of reinforcing steel components used could be modified easily and 
quickly to facilitate the investigation of a number of configurations. Two 
typical arrangements of reinforcing steel are illustrated in Figures 3 and 4. 
Typically, the steel elements examined were 5 ft. (1. 5 m) in length. The 
scan drive had a total traverse capability slightly greater than the length of 
the steel specimens so the specimens could be scanned from end-to-end if 
desired. By adjusting the end bulkheads on the simulated beam structure 
or the height of the magnet, the spacing between the magnet and/or probe 
and any particular steel reinforcing component could be adjusted to a 
selected value in the range of 0. 2 5 in. (0. 64 cm) to ^ 6 in. (15 cm). The 
probe was mounted with respect to the magnet such that the path along 
which the probe scanned a particular steel component could be varied in a 
transverse direction across the test bed; this permitted the probe to follow 
a scan path either directly below the steel component of interest or on a 
path off-center up to 'V 1 inch (2. 5 cm) off-center (by repositioning the 
magnet, greater off-center distances could be obtained). Figure 5 illustrates 
a typical arrangement for use in testing the influence of stirrup rebars 
on the magnetic signatures from simulated flaws; using the same basic 
approach, several different rebar reinforcing steel configurations were 
investigated. A comparison of Figure 6 with Figure 4 shows the manner in 
which a variety of pole tip configurations could be investigated for their 
influence on the signatures obtained from flaws. 



14 




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B. Experimental Approach 

The magnetic field investigations were conducted by inserting 
specimens containing manufactured flaws and unflawed steel sections in 
various configurations in the simulated beam specimen, applying a DC 
current to the electromagnetic, mechanically moving the simulated beam 
containing the test specimens past the magnet-probe head unit, and record- 
ing the signal output from the Hall-effect probe on a strip-chart recorder. 
Utilizing this general approach, the following parameters were explored 
on a preliminary basis: 

(1) Flaw size and configurational parameters including length, 
section loss, orientation of nonsymmetrical flaws and 
separation of simulated fracture surfaces; 

(2) Influence of adjacent unflawed steel members; 

(3) Scan path with respect to center line of flawed steel 
members; 

(4) Type of duct; 

(5) Type of reinforcing steel; 

(6) Transverse rebars; 

(7) Probe- to- specimen spacing; 

(8) Magnetizing field strength. 

Table 2 summarizes the test specimen configurations including flaw 
sizes and shapes used in the subject investigations. Many of the specimens 
listed in Table 2 are shown in Figure 7 including typical sections of flexible 
and rigid duct. Closeup views of flaw test sections for many of the speci- 
mens are shown in Figures 8, 9, and 10. The amount of cross-section 
removed by machining (simulation of cross- section loss by corrosion) in 
terms of percentage of the original bar cross -section has been calculated 
for each flaw type and tabulated for the corresponding specimen in Table 2. 
The results of the numerous experimental investigations are presented in 
the section which follows. 

C. Experimental Results 

Many typical recordings are presented in Figure 11 which illustrate 
typical signatures from duct, strand, bar, and rebars. A brief discussion 



20 



TABLE 2. 
TEST SPECIMEN (Manufactured Flaws) 

Flaw Configuration I 




■— b 

:c 






a(nom. ) 



5' - 1 




Flaw Configuration II 

1 





5' - 1 



Specimen 


Material 


Flaw Conf. 


a (in. ) 


b(in. ) 


c(in. ) 


r(in. ) 


% Area* 


A 


Strand, 7 -wire 


I 


0.5 


0.117 


0. 13 


— 


86 


B 


Strand, 7 -wire 


I 


0.5 


0.35 


6.0 


- 


43 


C 


Strand, 7 -wire 


I 


0.5 


0. 17 


6.0 


- 


86 


D 


Cold Rolled Bar 


I 


1.0 


0. 25 


0. 13 


- 


94 


E 


Cold Rolled Bar 


I 


1.0 


0. 80 


1.0 


- 


36 


F 


Cold Rolled Bar 


I 


1.0 


0.80 


6.0 


- 


36 


G 


Cold Rolled Bar 


- 


1.0 


— 


- 


— 





H 


A722, Type I 


II 


1.0 


0. 20 


- 


1.0 


14 


I 


A7 22, Type I 


II 


1.0 


0.50 


- 


1.0 


50 


J 


A7 22, Type II 


II 


1.0 


0. 50 


- 


1.0 


50 


K 


A7 22, Type II 


- 


1.0 


— 


- 


— 





L 


A7 22, Type I 


I 


1.0 







— 


100 


M 


Strand, 7 -wire 


I 


0.5 


2 wire 


variable 


— 


28 


N 


Strand, 7-Mre 


I 


0.5 


3. 5wires 


variable 


- 


50 


P 


Strand, 7 -wire 


I 


0.5 





variable 


- 


100 


Q 

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A7 22, Type I 

— 


- 


1.0 


— 




— 






* % Area lost (removed); % Area remaining = 100% - % Area lost 

NOTES: Typically, 5-ft. lengths of 2-5/8-in. diameter rigid and flexible duct 
were used to simulate post-tension configurations. 
1 inch = 2. 54 cm 



21 



Specimen 



Flexible 
Duct 



B 



C 



E 



D 



H 



Rigid 
Duct 




FIGURE 7. OVERALL VIEW OF TYPICAL TEST SPECIMENS 



22 






Specimen 



A 



C 






B 



FIGURE 8. CLOSEUP VIEW OF STRAND SPECIMENS WITH MANU- 
FACTURED FLAWS 



23 









Specimen 



D 



F 









FIGURE 9. CLOSEUP VIEW OF CRS BAR SPECIMENS WITH MANU 
FACTURED FLAWS 



24 



Specimen 



H 



FIGURE 10. 



CLOSEUP VIEW OF ASTM-A722 TYPE I AND TYPE II 
BARS WITH MANUFACTURED FLAWS 



25 





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26 



of this figure is in order to acquaint the reader with the data presentation 
format. The vertical excursion in each record is proportional to the 
magnetic disturbance generated at the probe by the specimen and any- 
attendant flaw conditions; generally, there is a large vertical excursion 
at each end of scan (see record C, Figure 11) caused by the demagnetizing 
effect of the specimen ends. The horizontal scale in each record is 
proportional to distance along the specimen, a typical scale of which is 
shown at the bottom of record A (Figure 11). 

Further examination of the records in Figure 11 will acquaint the 
reader with typical features of magnetic field response. Record A was 
obtained from a length of flexible duct containing no reinforcing steel 
within it and with the probe located 2. 5 inches (6.4 cm) below the center 
of the duct. Notice the relatively smooth line (other than the end effects) 
except for the signal departures pointed out by the arrows; these signal 
departures are caused by the fact that the flexible duct was not completely 
straight (the local spacing between the duct and the probe varies locally 
along the length of duct). Record B was obtained under conditions similar 
to record A except using a piece of rigid duct; notice the relatively 
straight base line with a periodic pattern of small bumps caused by the 
extra thickness of steel in the helically formed bead on the rigid duct. The 
signature insert within record B, obtained with a 1-inch (2. 54 cm) diameter 
section of ASTM A722 Type I bar within the duct, shows that as reinforcing 
steel cross- section is added inside the duct it tends to smooth the influence 
from the helical bead on the duct. Interestingly, and fortunately, the 
helical thread-like protrusions on the ASTM A722 Type II bar (such as 
tradename Dywidag) does not produce any significant background signal 
(see record C). Record D, Figure 11, illustrates the influence of a flaw 
and also rebars(transversely oriented to the test specimen such as for 
stirrups). The rebars give the same polarity of signature, e. g. , viewing 
from left to right downward going and then upward going, as the flaw; 
however, the amplitude of signature from the rebar is relatively small. 
The data in record E illustrates the combined effect of rebars along with 
a section of rigid duct but without reinforcing bar inside. It is pointed out 
that the distance from the probe to the rebar in the case of record E is 
considerably greater than that for record D; correspondingly, the signa- 
ture from the rebar is significantly less in the case of record E. Record 
F, Figure 11, illustrates the combined magnetic response from a section 
of rigid duct containing a bar specimen with a manufactured flaw and two 
rebars transversely placed above the duct to either side of the flaw. The 
results illustrated in Figure 11 are very encouraging in that a relatively 
minor response is obtained from configurational artifacts of the typical 
steel members as compared to response from manufactured flaws. 

To facilitate an overall view of the influences and interactions of the 
many test parameters on signatures from flaws, signature information from 



27 



the inspection records has been reduced and is presented in tabular and 
graphical form in this section of the report, and specific brief comments 
are made below about each of the presentations. 

Table 3 - Influence of adjacent unflawed material on the signal 
obtained from a flawed specimen. 

Results from specimen A indicate the influence of adjacent 
strands on the flaw signature amplitude is minimal. For 
specimen D the influence of flexible duct is less than that for 
rigid duct when adjacent bars are not present; the influence 
of neither duct is extremely significant. Specimen D results, 
however, do show the presence of adjacent bars on either 
side of the flawed specimen (4 in. or 10 cm spacing) do 
indicate a significant reduction in signal amplitude from the 
flaw; this reduction in signal amplitude may be caused by 
insufficient magnetization. The results from specimens E 
and F show a more pronounced effect of rigid duct on signal 
amplitude than for specimen D, perhaps because in the case 
of specimens E and F the flaws extend over a greater length. 
The apparent increase in flaw signal amplitude for specimen 
F inside a flexible duct is misleading and is probably the 
result of an inability to maintain the spacing between the 
probe and the specimen with sufficient repeatability for the 
"with" and "without" flexible duct tests. 

Table 4 - Influence of the transverse position of probe scan along 
the specimen (vertical distance from the probe to the specimen 
remaining constant). 

The results indicate approximately a 50% reduction in signal 
for the probe track 1-inch (2. 5 cm) off-center [probe-to- 
specimen vertical spacing of 1. 5 inches (3. 8 cm)]. The 
reduction in signal amplitude appears to be essentially 
independent of the presence of steel adjacent to the flawed 
specimen and essentially independent of the flaw configura- 
tion (note the dimensions for specimen C are significantly 
different than those for specimen A, refer to Table 2). 

Figure 12 - Signal amplitude as function of probe -to- specimen 
spacing. 

The results from specimens A, B and L tend to indicate the 
same general functional relationship and it is one which is 
strongly dependent upon the spacing. 



28 



TABLE 3 

INFLUENCE OF ADJACENT REINFORCEMENT 
ON FLAW SIGNATURE AMPLITUDE 



Specimen 


Relative 


Flaw Signature Amplitude 


No. Adj. Strands 


No 
Duct 


Rigid Duct 


Flexib! 


.e Duct 


No 
Adj. Bars 


Two 
Adj. Bars 


No. Adj. 
Bars 


Two 
Adj. Bars 





1 


2 


11 


A 


100% 


95% 


94% 


88% 


- 


- 


- 


- 


- 


D 


- 


- 


- 


- 


100% 


85% 


69% 


96% 


67% 


E 


- 


- 




- 


100% 


67% 


- 


- 


- 


F 


- 


- 


- 


- 


100% 


71% 


- 


111% 


_^™^™_™„ 



TABLE 4 

INFLUENCE OF TRANSVERSE LOCATION PROBE 

SCAN TRACK ON FLAW SIGNATURE AMPLITUDE 

(probe -to- specimen vertical spacing 1. 5 in. , 3. 8 cm) 



Specimen 


Relative Flaw Signature Amplitude 


Scan On- Center 


Scan 1 -in. Off- Center 


A Only 
A+l adj. strand 
A+2 adj. strands 
A+ll adj. strands 
C+ll adj. strands 


100% 
100% 
100% 
100% 
100% 


53% 
54% 
52% 
51% 
49% 



29 



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Figure 13 - Signal amplitude as a function of magnetizing current. 

The data for specimen A with a 1. 5 -inch (2. 8 cm) probe - 
specimen spacing shows a tendency towards saturation at 
higher magnetizing currents. The data for specimen B at a 
spacing of 2. 5 inches (6. 4 cm) and that for specimen A at 
3. 5 inches (8. 9 cm), particularly, indicates significant 
further increases in signal amplitude would be obtained 
with greater applied field strength. Other preliminary 
results to be presented larer in this section will indicate 
that by proper configuration of the magnetic circuit additional 
signal amplitude can probably be obtained without further 
increases in magnet power. 

Figure 14 - Signal amplitude as a function of flaw length. 

The results from specimens D, E, F and L show an extremely 
rapid increase in signal amplitude for flaw lengths from al- 
most zero up to the order of 1/8 inch (3 mm) and then a 
relatively slow additional increase in signal amplitude for 
greater flaw lengths. These results tend to indicate that 
deeply corroded regions would not have to extend over a 
great distance along the strand or bar to be detectable. 

Figures 15 and 16 - Signal amplitude as a function of loss of 
cross section. 

The results from specimens B and C, Figure 15, indicate an 
almost linear relationship between signal amplitude and 
percent area lost; on this basis, liberty has been taken and 
a straight line relationship assumed with the other curves in 
Figures 15 and 16. The data point corresponding to speci- 
mens I and J, Figure 16, indicate a tendency for lower signal 
amplitudes from flaws not symmetrically distributed about 
the reinforcement cross- section (compare the data point from 
specimens I and J with those from specimens E and F in the 
same figure. 

It is informative to now examine the records in Figure 17 because 
they indicate good sensitivity for detecting partial and total fracture in 7- 
wire strand even with relatively small separation of the fractured inter- 
faces. The data in Figure 17 were obtained with the flaw specimen in a 
matrix of 11 additional unflawed strands. Figure 18 illustrates similar 
results for bar-type specimen in rigid duct with a probe-to- specimen 



31 



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37 



spacing of 2. 5 inches (6. 4 cm). Records A and B at the top of Figure 18 
illustrate the very rapid increase of signal amplitude for separations up to 
approximately 1/8 inch (3 mm) as commented on in the previous discussion 
of Figure 14. Records C and D illustrate the fact that if the bar is in 
contact with the duct in the region of the fracture, the magnitude of the 
signature is not reduced. As a matter of fact, a comparison of records 
C and D with A and B, respectively (Figure 18), indicate an increase of 
signal; however, this is probably a result of the reduced spacing between 
the probe and the bar specimen since the position of the duct was not 
altered in the experiment. 

Figure 19, encouragingly, illustrates excellent detectability for 
post-tension bar fracture having end- separation with i) a significant 
cross- section of rebar directly over the fracture area (compare records 
A and B) and ii) good detectability of 0. 5-inch (1. 3 cm) separation with 
probe -to- specimen spacings as much as 6.5 inches (see record F, Figure 
19). Furthermore, a comparison of records D and E in this figure illus- 
trate that an almost 2 5% increase in signal amplitude was obtained by 
using flat plate-type pole tips versus the conventional wedge-type pole tips 
used almost throughout this investigation (refer to Figures 4 and 6 showing 
the two pole tip configurations). This preliminary indication of the in- 
fluence of pole tip design on flaw signal response suggests that significant 
further improvements should be possible with additional design investi- 
gations. 

Finally, Figure 20 presents magnetic signature results obtained in 
an attempt to inspect near the fractured end of one of the failed 1-3/8-in. 
(3. 5' cm) diameter bars (see Figure 1) from the Sixth South Viaduct 
structure in Salt Lake City. The fracture specimen used in this experiment 
is uniformly corroded around the entire bar circumference within about 
10 inches (25 cm) of the fracture; the corrosion rapidly tapers about the 
circumference and ends about 2 ft. (60 cm) from the fracture. The loss of 
section near the fracture is /v8-10%. It is evident in record A (Figure 20) 
that no signature from the corrosion is detectable; the large signal from 
the bar ends makes it difficult to detect signature trends from the much 
lesser effects of corrosion. Next, a 1-3/8-in (3.5 cm) diameter steel bar, 
20-in. (51 cm) length was placed adjacent to each end of the Utah sample 
to minimize the end "effect" signals (see record B, Figure 20). However, 
because of the unevenness of the ends, "gaps" of ~ 0. 05-0. 10 in. 
( ^ 1 to 2 mm) were present which produced the large signals observed. 
Record C, Figure 20, wherein the probe was scanned about 0. 25 in. (0. 64 
cm) from the bar surface shows response from local corrosion pitting 
(pointed out by the arrows); but again, effective results cannot be obtained 
any closer than about 6-8 inches (15-20 cm) from the fracture because of 



38 




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=End ofg 
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I"* - 2 in. typ. 



32 -in. length Utah fractured bar, in rigid duct, 2. 75 -in. 
probe-to-bar spacing. 




32 -in. length Utah fractured bar with 20-in. length of steel bar 
adjacent each end, no duct, 2. 0-in. probe-to-bar spacing. 




32-in. length Utah fractured bar, no duct, probe ^0.25 in. 
below bar surface. 



FIGURE 20. MAGNETIC SIGNATURES (3 Amp) FOR 32-IN. LENGTH 
FRACTURED BAR FROM SDCTH SOUTH VIADUCT, 
SALT LAKE CITY 



40 



end effects. Nevertheless, record B in Figure 20 further confirms the 
results previously shown in Figures 14, 18, and 19 - that very strong 
magnetic response should be obtained from fractured reinforcement even 
when inside steel duct and with thick concrete cover. 



(H 



IV. CONCLUSIONS 

The preliminary experimental investigation of the magnetic field 
method has established several important features about the capabilities of 
the method, namely: 

(1) Good overall sensitivity to loss-of- section. 

(2) Excellent overall sensitivity to fracture even with relatively 
small end -separation. 

(3) Relatively minimal degradation of signal response in presence 
of steel duct. 

(4) Presence of reinforcement adjacent to flaw has only a slight 
influence on flaw signal (if adequate magnetization is provided). 

(5) Configurational artifacts (helical band on duct, thread-like 
protrusions on Type II bar, etc. ) and structural features 
(rebars, bar-duct contact, etc.) have relatively minor negative 
influences on flaw detectability. 

(6) Probe-to-reinforcement spacing (both vertical and transverse) 
is a significant parameter influencing overall magnetic response. 

(7) Magnetizing field strength required is a function of steel section 
to be inspected and the distance from the magnet to the steel 
element under inspection. 

Importantly, corrosion specimens typical of field conditions, are 
needed to further evaluate the magnetic field approach and to realistically 
establish design parameters. Such specimens must have sections containing 
varying degrees of corrosion sufficiently far from the specimen ends to be 
useful. Additional experiments should be conducted using longer specimen 
lengths with realistic corrosion to confirm the results obtained from shorter 
specimen. Furthermore, experimental investigations should be conducted to 
explore approaches for more effective inspection in the vicinity of reinforce- 
ment ends. 



42 



V. RECOMMENDATIONS 

On the basis of the results from the literature assessment of 15 NDE 
methods and the preliminary experimental investigations conducted using the 
magnetic field method, it is recommended that the magnetic field method be 
selected for development of a conceptual design followed by detailed design 
and fabrication of a breadboard unit. The conceptual design, however, should 
include sufficient laboratory work to realistically specify the overall concept 
in terms of power requirements, weight, scanning and tracking features, etc. 
On this basis, it is crucial that Tasks D, E, and F be more closely inter- 
related than originally envisioned under the subject contract. While the 
overall estimated effort required in Tasks D, E, and F (combined) remains 
the same at this time, it is recommended that Task D be extended to 2-1/2 
months total in parallel with Task F and that Task E extend to end of 
Task D. It is estimated that the funds required in Task D will be approxi- 
mately $3, 000 greater than originally allocated and that the funds required 
in Task F will be diminished by an equal amount. The design review would, 
of course, be scheduled at the conclusion of Task E. 



43 



APPENDIX A 
DESCRIPTION OF METHODS 

ACOUSTIC EMISSION 

Acoustic emission is a measure of the energy released as a solid 
material undergoes plastic deformation or fracture. In particular, it has 
been shown that the sources of acoustic emission from metals may include 
microslip(4), twinningw), martensitic phase transformations^ ', micro- 
crack nucleation('), and crack propagation"'. Part of this energy is 
converted into elastic waves which propagate through the material and 
which can be detected by the appropriate transducers. The first basic 
studies were performed in the late 1940' s and early 1950' s in the United 
States and Germany. \°»'> One of the earliest reported engineering applica- 
tions of acoustic emission was in the surveillance of Polaris missile chambers 
at Aerojet General Corporation in 1964, during hydrostatic testing. '*"' 

The bulk of recent studies has approached the problem of stress wave 
emission from one or the other of two extremes of the deformation spectrum. 
At one end, Fisher and LallyW have been able to show conclusively that 
recognizable acoustic emission can be obtained from preyield dislocation 
bursts which generate a total plastic microstrain of only 1 X 10" . In some- 
what related work, Frederick, et al. (H»12) have succeeded in relating 
variations in acoustic emission characteristics of a variety of metals and 
alloys with the variations in their stacking fault energies, which strongly 
affect dislocation mobility. At the other end, investigations have been based 
upon the much higher amplitude acoustic waves generated by macroscopic 
fatigue cracks, ^ 0. 25 in. (0. 64 cm) in length in precracked specimens or 
in pressure vessels. \^~^» *-3) The latter work has been particularly related 
to the problems peculiar to nuclear pressure vessel design. 

Usually acoustic emission is limited to those flaw-detection problems 
where cracks exist and loading of the crack region sufficient to produce 
crack extension is possible. Such a procedure necessarily presumes know- 
ledge of the prior load history on the structure and a practical means for 
loading a structure which does not in itself damage the structure. Analysis 
of acoustic emission data from a complex structure such as a bridge is 
particularly difficult because of the emission from sources within the structure 
other than fatigue cracks (welds, bolts, rivets, joints, etc.). Recent field 
experiments conducted under an FHWA contract showed that on a steel bridge 
there was considerable low -level noise over a period of several weeks and 
that the use of multiple transducers with logic circuits provided at best a 
marginal indication of response from fatigue cracks known to be in the region 
being monitored. Whether the fatigue cracks were actually extended during 
the monitoring period was not confirmed since no other independent measure- 
ments or observations were reported. 



45 



Harris investigated the use of acoustic emission monitoring for the 
cables on the lift span of the Dunbarton Bridge. ' ' In this work it was 
necessary to apply a load to the cables by a transverse force to stimulate 
acoustic emission. No results were confirmed that would indicate the 
potential of this method for detecting reinforcing steel deterioration. 
However, it was indicated that the attenuation between strands of the cable 
was high with a value of 24 dB being measured for adjacent strands. In 
this case, the term strands is used for the individual wires which make up 
each cable. Also, measurements of attenuation along the cable ranged from 
1 to 5 dB/inch. 

v Laboratory experiments conducted using acoustic emission monitoring 
of wire -rope cables has shown that at high sensitivity settings the emission 
provided "ample warning of impending strand failure". Also it was indicated'***' 
that a one-to-one correlation was obtained between the number of broken 
wires and the number of events observed at a particular gain setting. During 
this investigation a variety of experiments were conducted; however, the 
cables were only 5-1/2 inch (14 cm) long between connections and the cables 
were 1/4 inch (0. 64 cm) diameter, 7 x 19 stainless steel and 6 x 19 improved 
plow steel with wire rope core. Extraneous sources of noise included the 
end attachments, rubbing at one cable on the other, etc. There was no 
indication of the method by which the transducers were mounted for sensing 
the acoustic emission. 

The relatively recent work of Bickle* ' explored the application of 
acoustic emission methodology for civil engineering applications and does 
indicate interesting potential applications. In one series of experiments 
acoustic emission from concrete beams without rebars and with rebars was 
conducted. While there were indications that differences in the emission 
were observed, the effort was primarily exploratory and does not provide a 
basis for knowing repeatability, variation from specimen to specimen, etc. 
It was suggested that the method could possibly provide a method of sensing 
rebar debonding in concrete elements. 

Personal communications from the Utah State Highway Department 
indicated that fracture of a reinforcing strand was reported when a loud 
cracking noise was heard by people in the vicinity of a bridge in which several 
reinforcing strands are known to be fractured. 

In summary, the foregoing indicates promise of acoustic emission 
methods for providing information on condition of reinforcing strands. How- 
ever, the relatively high attenuation of the strands and the concrete overlay, 



46 



numerous potential extraneous noise sources, the necessity of loading beams 
to stimulate emission, and the fact that the method would probably not provide 
an indication of strand deterioration until most of the section is lost places 
acoustic emission low in the assessment ranking. However, acoustic 
emission monitoring should not be completely discounted, since arrays of 
inexpensive transducers permanently affixed to each bridge element and 
transmitting data to central receivers for overall monitoring of the bridge 
structure, could possibly become economically and operationally practical 
in the future. 

EDDY CURRENT 

Eddy current methods depend on the electromagnetic induction of 
electric currents in metals by a coil carying alternating currents. These 
eddy currents cause the impedance of the exciting coil or any pickup coil 
in close proximity to change as a function of the material characteristics. 
A number of factors influence the eddy current characteristics, including 
magnitude and frequency of the exciting alternating current, electrical con- 
ductivity, magnetic permeability, configuration of the part, relative position 
of the coils and the part, and more importantly, the presence of discon- 
tinuities or inhomogeneities in the material. ' A °' Eddy current inspection 
techniques have been widely used, and the American Welding Society' "' 
describes the application, principles of operation, advantages, and pre- 
cautions for using eddy currents to examine welds. Automated weld followers 
and inspection devices are described by Smith and McMasters' 1 '' and 
ForsterUO). 

There are two general approaches to eddy current testing. (*°» 21 » 22) 
The first of these may be called the single-coil approach. Here, properties 
of the test article are inferred from the effects on relative amplitudes and 
phases of the voltage and current flowing in a wire coil (the "probe"). The 
second approach is to introduce a second sensor. Properties of the test 
article are inferred from the effects of eddy currents on the secondary sensor. 
In the above approaches, the sensors may be constructed for either "absolute" 
or "differential" sensing. Recently, a new eddy current instrument, referred 
to as the magnetic reaction analyzer'* '', has been developed which utilizes 
a Hall effect element, instead of coil type probe, to sense the magnetic 
reaction fields. When an inspection is performed at a fixed test frequency, 
any material discontinuity that alters the conductivity or thickness (geometry) 
of the item under test will be reflected in the magnetic field sensed by the 
Hall probe. In practice a test frequency is chosen to penetrate the material 
to the desired inspection depth. The depth to which inspection can be per- 
formed may be adjusted, using a variable of test frequency, and when necessary 



47 



the excitation coil diameter. On large parts large-area excitation coils 
facilitate deep penetration and can tolerate what is considered unusually 
large lift-off variations (1/4 inch (0. 64 cm) in steel). However, it is doubt- 
ful that the method is useful for inspecting reinforcing steel with the amount 
of concrete coverage present on bridge structural beams. Other influencing 
factors such as the possible presence of magnetite in the concrete agregate 
could further reduce the useful sensitivity of the instrument. 

In general, performance of an eddy current approach would probably 
be similar to that obtained from pachometer-type instruments'"' 24) USe( j 
to locate reinforcing steel in bridge decks. Such instruments utilize low 
frequency magnetic field excitation coupled with a impedance -change measure- 
ment. Experience has shown such instruments to be influenced by the presence 
of magnetite; furthermore, the sensitivity of such instruments to loss of steel 
due to corrosion or fracture has been rated poor. 

ELECTRICAL RESISTANCE OF CONCRETE 

It has been shown that electrical resistivity measurements can be 
used as a nondestructive method for the determination of pavement thickness 
and depth to reinforcing steel. (^o) -j^ re sistivity test, used for many years 
in subsurface exploration work throughout the country, involves a measure- 
ment of the resistance to the passage of an electric current through the 
material undergoing testing. The test is made by using four electrodes 
equally spaced in a line on the surface of the material being tested. The 
nature of the test is such that the effective depth (penetration of the applied 
current) is approximately equal to the electrode spacing for a particular 
setting of the electrodes. 

Four small plastic tubes, plugged with stiff clay and filled with a 
saturated solution of copper sulphate into which a copper wire is inserted, 
are used in the test. The clay, with the help of a wetting of the concrete 
surface with ordinary tap water, provides for a suitable contact for the 
electrical circuits with the pavement surface. Most materials have a char- 
acteristic resistivity and the test procedure, which involves all material 
from the surface to the depth involved for a particular electrode setting, will 
produce resistivity values which will trend toward higher or lower resistivity 
values depending upon the fundamental resistivity of the second layer (the 
base course in the case of pavement thickness tests). 



The presence of a conducting material such as steel reinforcing bars 
affects the resistivity test as the depth of test reaches the layer of steel 
reinforcement. Results have shown an average variation in the resistivity 



48 



measurements of depth to steel of 8. 6% (0. 35 inch, 0. 90 cm) for 110 tests. ' 2 °' 
One factor likely affecting the measurement of depth to reinforcing steel is 
the failure to always locate a test point directly above a steel bar. 

Another aspect of resistivity measurements is based on the fact that 
concrete, like soils, normally has a fairly high resistance; and one might 
expect if low resistances were measured, it might be an indication that a 
corrosive environment existed within the concrete' 2 ^'. Observations have 
indicated that when the resistivity is above 60, 000 ohm-cm, no corrosion 
occurred; for resistivities below 60, 000 ohm-cm, corrosion was detected-- 
generally, the lower the resistivity the greater the corrosion of the steel. 
Because water content affects the resistivity, such measurements need to 
be made while the spans were wet. Also, measurements must be made on 
a grid pattern to be certain that local anodic areas are being monitored. 
The resistivity of anodic areas was found to be always lower than that of 
cathodic regions. It has been quoted'^) that- -"It should be strongly em- 
phasized that this resistance approach would need considerably more study 
before it could be recommended for determining when serious corrosive 
conditions existed in a bridge structure. " Importantly, the resistivity 
approach in concrete does not directly assess the condition of the steel but 
only indicates that corrosive conditions are present. 

ELECTRICAL RESISTANCE OF STEEL 

If penetration of the concrete (and duct) in a prestressed member is 
considered, a measurement of the resistance of the steel might be a viable 
method for determining corrosion or fracture of the steel. Such an approach 
has been proposed to determine continuity'^'' ^°> by making electrical 
contact to the ends of reinforcing bars and measuring resistance of the bars 
by forcing a known current through the bar and measuring the resulting 
potential drop across the bar. 

To establish contact with the steel bar, a hole was drilled through the 
concrete beam and the duct after which a small spot, approximately 1/4 inch 
(0. 64 cm) in diameter and 0. 030-0. 040 inch (0. 75-1. mm) deep was drilled 
on the surface of the bar for electrical contact. Bronze electrodes were 
tightened against the bar by means of threaded devices. A DC current 
(10 ampere) was forced through the bar and monitored with an ammeter. 
If loose or faulty contact occurred, the current reading would fluctuate; 
adjustment of electrodes was made until a constant current was obtained. 
Bar resistance was determined by potentiometer measurement of the re- 
sulting potential drop across the bar. Probably the greatest contributing 
factor to variations in measured potential from bar to bar was differences 



49 



in contact between the electrode and the bar. The contact varied because of 
differences in hole location, va rations in size and shape of the spots drilled 
on the bar, and variations in the angle that the hole was drilled. 

Although it was initially reported* 2 ' ' that the method showed some 
potential for detecting reduced sections in bars, laboratory investigation 
of the method showed that the spread in the data was so large as to make the 
method impractical for determination of reduced section dimensions in steel 
bars< 28 ). 

Resistance calculations using a simplified model tend to support the 
conclusion that the method is insensitive. For example, the presence of a 
3 inch (7.6 cm)-long section with only 6% of the cross-section remaining in 
a 40 foot (12. 2 m)-length of bar increases the total resistance only 10%; 
importantly, a 10% variation in resistance could result from dimensional 
tolerances alone on the bar. Similarly, a 50% reduction in bar cross- section 
over a two foot-length would only increase the resistance approximately 10%. 
The complicating factors of penetrating the concrete (and duct) and obtaining 
reliable contact as well as the influencing factors of contact between the bar 
and stirrups and /or duct and bar tolerances do not make the method appear 
promising. 

ELECTROMAGNETIC INSPECTION METHODS 

Electromagnetic techniques applicable to the problem of indirectly 
inspecting the integrity and load strength of prestressed concrete beams are 
of two basic types: 

(1) A high-resolution concrete-penetrating radar capable of 
indicating the physical status of steel rods and cables; and 

(2) Electromagnetic response to electrical nonlinearities asso- 
ciated with corrosion products resulting from deteriorated 
steel members. 

The first method is a means for physically locating the sizing sections of the 
steel components of the beam, within practical limits of resolution, for the 
purpose of directly measuring changes in metallic cross-section and possibly 
detecting fracture conditions in the load carrying steel elements. The second 
method pertains specifically to the corrosion status of the steel elements and 
would utilize this information as an indirect empirical measure of the integrity 
of the reinforcing steel components of the beam. 



50 



In considering the applicability of these two techniques to the inspec- 
tion of prestressed concrete beams, neither method is applicable to beams 
having tensioned steel rods or cables contained inside metallic conduit tubes. 
Electromagnetic energy appropriate for reflection or nonlinear response 
from a metallic structure cannot penetrate such conduit materials and, 
hence, the methods offer only a means for inspecting the physical status of 
the outer surface of the metal conduit, not the reinforcing steel member 
contained inside. If the conduit tubes used in the beam are non-metallic, 
both of the electromagnetic inspection methods mentioned above can readily 
penetrate such enclosures and may potentially function in the same manner 
as if the conduits were absent. 

For the case where the tensioned steel members are enclosed in 
metallic conduits, a third electromagnetic inspection method is suggested, 
offering the capability of detecting voids in the grouting used to seal the 
enclosed reinforcing steel members. This method utilizes the fact that the 
steel rod or cable enclosed within a metallic tube forms an approximate 
coaxial structure crudely comparable with that of an electromagnetic coaxial 
transmission line. Thus, by applying time domain reflection testing con- 
cepts to such an imperfect transmission line the relatively large discontinuties 
resulting from air voids in the grouting injected to fill the annular space 
between the cable and the conduit may be detected and located in approximate 
position along the beam structure. Grouting voids are known to be highly 
susceptible locations for corrosion deterioration of prestressed concrete beams. 

Electromagnetic Reflection Method 

Electromagnetic reflection methods applicable to concrete beams are 
similar in concept to conventional radar ranging techniques with the exception 
that the atmospheric propagation medium is replaced by the concrete and the 
radar target sizes and detection ranges are reduced by several orders of 
magnitude. Electromagnetic wave propagation in concrete differs from pro- 
pagation in air in that the propagation velocity is reduced by about 40-50% 
as a result of the higher permittivity of concrete and target range -dependent 
and frequency-dependent absorption losses occur because of the higher con- 
ductivity of concrete. The first of these differences is advantageous to small 
target detection and resolution because the wavelength of the target illuminating 
and reflected waves is reduced in direct proportion to the reduced propagation 
velocity. In regard to conductivity effects, electromagnetic absorption in 
dry concrete is relatively small, permitting useful penetration depths of 
several meters at wavelengths of a few centimeters (operating frequencies up 
to about 6 GHz). However, the presence of moisture and conducting mineral 
content in the concrete may reduce practical penetration depths to about one 
meter or less at wavelengths of a few centimeters. 



51 



Depth and size resolution of imbedded steel rods or cables is depen- 
dent upon the electromagnetic wavelength in the concrete. Thus, by reducing 
the operating wavelength (requiring increased operating frequency and signal 
bandwidth) the observed target reflection detail can be improved. This 
characteristic of the electromagnetic inspection method cannot be extended 
without limit, however, because of the frequency-dependent absorption 
effects in the concrete medium. That is, as the operating frequency is 
increased to obtain improvement in resolution, the useful penetration depth 
obtainable with a given operating source power is reduced and therefore the 
ability to adequately illuminate and detect the internally imbedded steel rod 
targets will ultimately impose the upper limit on operating frequency and 
resolution. 

The radar cross-section of a cylindrical metallic reflector is depen- 
dent upon its diameter in terms of the illuminating wavelength. When the 
incident E-field is parallel to the axis of the cylinder, circulating currents 
are induced parallel to the axis and strong reflections occur. In this case, 
the backscatter cross- section per unit length is approximately equal to the 
illuminated half-circumference of the cylinder for cylinder diameters equal 
to the illuminating wavelength or larger. Because of the longitudinal circu- 
lation currents which are established on the cylinder with parallel E-field 
polarization, the reflections remain strong even when the cylinder diameter 
is small compared with the wavelength. When the incident E-field is per- 
pendicular to the cylinder axis, the backscatter cross-section per unit length 
is approximately the same as that for parallel polarization orientation for 
diameters equal to or larger than the wavelength, but decreases rapidly 
(i. e. , as inverse diameter cubed) for cylinder diameters smaller than the 
wavelength. 

In regard to these radar cross -section characteristics, strong reflec- 
tions may always be obtained from rods and cables which are thin compared 
with wavelength when the incident E-field is parallel to the rod axis; however, 
when the reflected signal strength is to be a measure of the effective cylinder 
diameter, the illuminating wavelength must always be equal to or less than 
the rod diameter being observed. Therefore, the polarization orientation of 
the incident wave is not important from a reflection cross-section viewpoint. 
This fact simplifies the practical scanning procedures required to inspect a 
concrete beam and will permit the radar transmitter and receiver antennas 
to be operated in a cross-polarized orientation to eliminate system self- 
interference feedover and avoid reflection responses from planar surfaces 
such as the air-concrete interface. 



52 



The electromagnetic inspection frequency required to yield a radar 
reflection whose strength is proportional to the diameter of a cylindrical 
rod target is 

f = !^k_ GHz 



D 



cm 



where: ^cm = ro< ^ diameter in centimeters; 

k = a numerical factor indicating the rod diameter in 

wavelengths where k ^ 1 . 

Thus, for k = 2 the radar frequency must be 12 GHz for D cm = 2. 5 cm and 
24 GHz for D cm = 1. 25 cm, corresponding to the diameters of typical steel 
reinforcing rods and cables, respectively. Penetration losses in concrete 
at these frequencies are estimated to be about 0. 10 dB/cm indicating that a 
radar system having a total transmission loss capability of about 40 dB will 
be required to yield a useful reflection signal (20 dB signal-to-noise ratio) 
suitable for determining the relative diameters of typical reinforcing steel 
elements embedded 10 cm in concrete. Using small microwave antennas 
this performance capability will require a microwave source power of about 
10 dBm. 

The highest operating frequency and the total transmission loss 
capability estimated for the radar reflection method is considered to be 
attainable within the present state-of-the-art and can be achieved using 
either short pulse or wide bandwidth FM-CW radar system techniques. 
The accuracy of the method will depend largely upon the uniformity of the 
absorption losses within the concrete since variations of this parameter 
cannot be directly distinguished in the observed reflection signal strength. 
However, by means of signal analysis methods which evaluate the backscatter 
signal versus frequency, the variability caused by inhomogeneous absorption 
effects may potentially be processed out of the inspection data to yield infor- 
mation related only to the steel rod diameter and its embedded depth within 
the concrete beam. 

Nonlinear Electromagnetic Response to Corrosion Deterioration 

Electromagnetic reflections from conducting metallic targets may be 
interpreted as electromagnetic reradiation from the structure as a result of 
circulating currents induced in it by the incident wave. Conventional radar 
systems operate using a receiver tuned to the frequency of the transmitted 
signal since the predominant currents induced in a metallic target have the 
same frequency as the incident wave. However, if the metallic target has 



53 



nonlinear electrical properties, as may be caused by corrosion oxides on 
its surface, then the induced circulating currents flowing through the non- 
linear conduction paths will be distorted resulting in the generation of 
harmonics and intermodulation products. These new circulating current 
components, produced as a result of corrosion effects, will also reradiate 
from the structure and may be detected if the receiver is properly tuned for 
their reception. 

Since the oxide coatings causing the nonlinearities will not necessarily 
rectify the induced currents, the bilateral nonlinear conduction effects will 
tend to generate odd-order distortion products. Thus, for an oxided metallic 
target illuminated by an electromagnetic wave, a small portion of the trans- 
mitter-induced energy will be converted to third- and higher odd-order 
harmonics and related odd-order sum and difference intermodulation products. 

When this nonlinear response inspection technique is applied only to 
indicate the presence of corrosion, ie. , the embedded depth and size resolution 
of the steel reinforcing members are not measured, the electromagnetic system 
need not operate at the high microwave frequencies required for the reflection 
inspection method described earlier. Instead the nonlinear response method 
can utilize CW electromagnetic transmission rather than pulsed or FM 
operation. The use of a continuously transmitting system can be most effec- 
tively implemented using a two-frequency system whereby the corrosion- 
related nonlinear distortion frequency is an odd-order intermodulation product 
not related to any of the residual harmonics which may be emitted directly 
by the CW source. Also, because a lower operating frequency will be 
employed, the incident wave would be linearly polarized parallel to the 
axis of the steel rods being inspected. 

The practical feasibility and accuracy of this nonlinear response method 
for assessing corrosion status of steel reinforcing rods and cables is not 
predictable on the basis of presently available information. The method is 
basically an impirical measure of corrosion effects which may vary with 
different materials and environmental conditions. Laboratory tests have been 
performed in the past to confirm the existence of nonlinear reradiation as a 
result of natural corrosion oxides present on metal structures. Further, this 
effect is often a noticeable form of radio communication interference aboard 
ships and other locations where transmitters and receivers may be operating 
simultaneously on appropriately different frequencies. 

Successful implementation of this inspection method will require that 
the transmitting source be thoroughly free of distortion frequency products 
intended to be observed from the nonlinear effects of the corroded steel elements 



5k 



and that the receiver have exceptionally good linearity so that interfering 
distortion frequency products are not generated in the receiver as a result 
of the strongly reflected fundamental signal. Additionally, no nonlinear 
metallic structures or electrical connections should be associated with the 
source or receiving antennas or extraneously located in the same vicinity 
as the inspection system. 

Time Domain Reflection Inspection 

Time domain reflectometry, as conventionally applied to electromag- 
netic transmission lines, may be a practical means of locating grouting voids 
and complete breaks in steel rods and cables contained in metallic conduit 
enclosures. The approximate coaxial structure of this type of concrete beam 
reinforcement configuration may potentially support the propagation of guided 
electromagnetic energy in the form of TEM waves. When an electromagnetic 
impulse is introduced at a given location on a transmission line structure of 
this type, distinctive reflections are produced by any variations in the char- 
acteristic impedance of the line. With a general knowledge of the conduit 
tubing internal diameter and the diameter of the steel reinforcing rod or 
cable which it contains and a typical value of the relative dielectric constant 
of the grouting material, the nominal impedance and propagation velocity of 
the coaxial structure can be determined. Analysis of the amplitudes, polarities 
and time delays associated with the observed impulsive reflections will yield 
sufficient information to determine the presence of air voids in the grouting, 
the length of such voids, and the approximate positions of the voids along the 
concrete beam. 

While this inspection technique cannot reveal direct information on 
the physical status of the load-carrying steel members of concrete beams, 
the detection and size assessment of grouting voids as described above may 
yield useful data which could guide the application of other more specific 
inspection processes. Additionally, since the time domain reflection tech- 
nique may be applicable to cable and conduit structures containing uncured 
grouting materials, this measurement method may be well suited for use in 
monitoring the initial grouting process to ensure that no voids remain in the 
reinforcement conduits of beams used in newly constructed highways and bridges. 

Application of this method will require separate electrical contact 
with the conduit tubing and the enclosed steel rod or cable. Access to these 
elements may be gained by an appropriate drilling process capable of inter- 
cepting the embedded conduit and penetrating the conduit to permit electrical 
contact with the internal load-carrying steel member. 



55 



HALF-CELL POTENTIAL 

In an article ^ > on the repair of spalling bridge decks, John 
Kliethermes states. . . "With a high degree of confidence, it can now be 
said that most spalling results from the expansive forces associated with 
corrosion. It can also be said that the predominant cause of corrosion 
is the salt that is absorbed by the concrete" and that "the corrosion of 
reinforcing steel is the result of an electrochemical process whereby 
corrosion cells are created by variable amounts of chloride, oxygen and 
moisture along the length of the reinforcement. These cells produce 
a flow of electric current between two half -cells, the anode and the cathode. 
The corrosion cell may be minute with the anode and cathode microscopically 
spaced, such as seen when steel corrodes in air, or they may be spaced 
several feet, as frequently found in bridge decks. " 

The electrical potential difference between the anodic half-cell 
and cathodic half-cell may be measured via a voltmeter. However, the 
difference in measured voltage between the two unstable half-cells is 
not too meaningful because the electrical activity will change as the 
electrolyte changes. Therefore, in order to compare readings over a 
period of time, a standard reference half-cell that retains a constant 
electrical potential must be used. The reference most commonly used 
is a copper/copper sulfate half-cell. It is comprised of a plastic tube 
with a porous plug inserted in one end, a pure copper rod, and a solution 
of saturated copper sulfate. The plastic tube is filled with the copper 
sulfate solution, and then the copper rod is inserted. The copper rod is 
stable in a copper sulfate solution, and therefore, its potential remains 
constant regardless of changing conditions. To compare the electrical 
potential of the reference half- cell to that of steel embedded in concrete, 
the (half-cell) copper electrode and steel must be connected. This is 
usually accomplished by making a positive electrical connection to the 
top mat of reinforcing steel and by providing a moisture junction through 
the concrete between the copper rod and the reinforcing steel at the point 
where the potential value is to be determined. A voltmeter is placed in 
the completed electrical circuit to measure the electrical potential 
difference. 

The values of potential difference measured can vary with variations 
in conductivity of the concrete and the means for making contact to the 
steel . Even wider variations have been observed when measurements 
are made without direct electrical connection to the steel' 4:J '. However, 
the same potential gradient contours (but not potential levels) have been 
noted regardless of whether or not direct connection was made to the 
steel(43). 



56 



The best measure of half-cell potential is via direct connection 
to the steel and there is generally good agreement that a potential difference 
of -0.35 volt or greater (more negative) is an indication of corrosive 
activity. In practice, however, potentials have been noted to fluctuate 
between active and passive levels over a period of weeks' '. It is, 
therefore, difficult to ascertain the condition of reinforcing steel by a 
single potential reading; in any case, the method is not quantitative unless 
a potential history is maintained and a rate of corrosion is predicated. 
Application of the half-cell potential method to bridge structural members 
vs. decks also involves other implementation considerations such as potential 
probe orientation difficulties, adequate wetting of concrete members, 
achieving electrical connection to steel, etc. The method will not 
distinguish between corrosive environments surrounding a duct vs. the 
reinforcing steel in the duct. 

HOLOGRAPHY 

Optical interference holography consists essentially of illuminating an 
object to be examined with a portion of the beam from a coherent optical 
radiation source (laser) causing the scattered beam from the object to 
interfere with a reference beam obtained from the same source and 
recording the resulting interference pattern by means of a photographic 
emulsion which is subsequently developed. The resulting negative is 
a diffraction pattern, and, when it is reilluminated with a suitable coherent 
light source, a three-dimensional "like" image of the original object is 
reconstructed. If the reconstructed image is superimposed on the original 
object, it is then possible to observe interference fringes in those regions 
where small displacements exist on the surface of the object which did not 
exist at the time the hologram was made. This two-beam method has been 
credited to Leith and Upatnieks' ''. In many cases, a second approach is 
used which consists of a double or possible multiple exposure of the 
emulsion to the irradiation scattered from an object when the object is 
successively photographed in one position and then in a slightly distorted 
position (as that caused by an applied mechanical load or a temperature 
change). Re illumination of this multiple exposure then reconstructs the 
image with interference fringes appearing which correspond to the 
relative displacement of the object's surface for the two exposures. 
Optical interference holography has been used to detect flaws in many- 
types of bonded structures^ '. These approaches have been used also 
to study the deformation of components under load^- 1 -', as well as fatigue, 
fatigue cracking, and stress-corrosion cracking^ ' '. 

Regardless of application, the method works by comparisons of 
surfaces in two different states of stress or load. Because it is a surface 
effect, holography cannot directly be used to locate corroded or fractured 
tensioning members imbedded in prestressed concrete. Indirectly, it may 



57 



be used to identify areas on the exposed surface of the concrete which 
contain high or anomalous stress distributions^ '. Areas so defined 
may reflect the presence of a broken tensioning member beneath the 
surface, provided the concrete structure in question can be loaded 
sufficiently to cause a change in the surface stress distribution. 

In practice, the method is at best difficult to instrument in the 
field. The bridge environment dictates the use of extremely high power 
laser pulses of exceedingly short duration for creation of the hologram. 
Such lasers are both expensive and bulky in addition to being somewhat 
of a safety hazard. Also, the interference fringe pattern obtained will 
be very difficult to interpret, as it will reflect the total change in stress 
distribution, containing components attributable to mounting geometries 
and loading in addition to the components attributable to the steel 
tensioning members. 

MAGNETIC FIELD 

The common principle of magnetic field methods is the detection 
of magnetic field anomalies in a relatively uniform magnetic field where 
these anomalies are caused by material inhomogeneities such as voids, 
inclusions, cracks, chemical desegregations, etc. Two broad categories 
of problems exist. In one, a relatively uniform magnetic flux is established 
in a region essentially filled with a ferromagnetic material, the region is 
then scanned with a sensitive, high resolution magnetometer (probe) to 
detect local anomalies caused by varying permeability produced by voids, 
discontinuities and locally stressed regions. In the other, a relatively 
uniform magnetic field is established in an extended volume containing only 
a small or local region of ferromagnetic material, such as magnetometer 
surveys for submarines. In the reinforcing strand deterioration or fracture 
problem, the steel is only a fraction of the volume being examined and yet 
it is desirable to detect anomalies caused by local flaws in the steel. 
Accordingly, it is a case intermediate between the preceeding cases. 

Several instruments'"^', for example Pachometer, Covermeter, 
Ferrometer, Pribor IZS and FEMETR, operating on magnetic principles, 
have been developed for measuring either the distance of reinforcing bar to 
the surface of concrete when the diameter of the reinforcing bar is known, 
or to measure the diameter of a bar when the thickness of the concrete covering 
the bar is known. FHWA has made rather extensive evaluations of the Pacho- 
meter principle and has concluded that the accuracy "and reliability of the 
system proved excellent"'^ 4 '. The instrument, of course, is designed for 
making measurements of concrete cover over reinforcing bars and in dis- 
cussions with one of the FHWA investigators, it was indicated that the method 
probably was incapable of detecting loss of section from deterioration. One 
of the other instruments (not identified) was investigated'"-*' and it was indi- 
cated that while the instrument could measure cover and also obtain a 

58 



reasonable measurement of rod diameter when a single rod was involved, 
proximity of adjacent rods in a typical deck or beam configuration produced 
significant errors. For example, for a 1-3/8 inch (3. 5 cm) rod diameter 
with a 2 inch (5 cm) concrete cover, the indicated rod size was 9/16 inch 
(1.4 cm), and for a 1-3/8 inch (3.5 cm) diameter rod with 2-1/2 inch (6.4 cm) 
coverage, the indicated size was 5/8 inch (1.6 cm) diameter. Accordingly, 
it is judged that this particular magnetic field instrument operating from the 
basis of "comparing the fixed electromagnetic characteristics of a reference 
transformer with the variable ones of a measured transformer (probe)" offers 
little promise for detecting reinforcement steel deterioration. 

As indicated earlier, the present problem is generically between that 
of a region completely filled with ferromagnetic material and of a region with 
only a small fraction of magnetic material. Since extensive prior investiga- 
tions!^ -61) } both experimental and analytical, demonstrated excellent 
sensitivity and resolution of magnetic perturbation for the detection of minute 
flaws and fatigue cracks, it is possible that the magnetic field methods when 
adapted for the reinforcing steel deterioration problem can provide good 
detection capability. The unknown influence of distance between the magne- 
tizing source and the material to be magnetized, limited fill factor of the 
steel occupying the region being magnetized, and complexities of the steel 
configuration would require extensive investigation before capabilities and 
limitations and advantages and disadvantages of the method could established. 

MOSSBAUER EFFECT 

The possibility of using the Mossbauer effect to assess the state of 

{ 3M 
corrosion of steel has been proposed. The Mossbauer effect* ' is the 

resonant absorption of gamma rays by atomic nuclei. The energy (or wave- 
length) of nuclear gamma rays is extremely well defined. Very small effects 
upon nuclear energy levels, such as those produced by the magnetic and 
electric fields arising from orbital and conduction electrons, can shift the 
nuclear energy levels by an amount which destroys resonant absorption. This 
can be counteracted by shifting the energy of the incident gamma rays through 
the Doppler effect produced by putting the gamma ray source in motion. The 
resonant absorption rate as a function of source velocity produces a Mossbauer 
"spectrum", the features of which may be interpreted in terms of changes in 
the magnetic and electric field environment of the target nuclei. Because 
the local electric field environment of iron nuclei in corrosion products differ 
from the environment in undamaged steel, it is, in principle at least, feasible 
to employ the Mossbauer effect in measurements of corrosion. The equip- 
ment required is basically a radioactive source of gamma rays (Co- 5 ' in this 
case), a servo-controlled velocity transducer to move the source, a gamma 
ray detector, and a multichannel pulse height analyzer. No field-worthy 
equipment as such appears to be available, and this approach to fatigue crack 
detection will not be considered for further evaluation in the present program. 

59 






RADIOGRAPHY 

Radiography is based upon the attenuation of a beam of penetrating 
radiation by a specimen^^'. Discontinuities in an otherwise homogeneous 
specimen are revealed by the change in attenuation which they produce. 
Film radiography uses photographic films to record a "shadowgram" or 
radiograph of the transmitted radiation. The detectability of a flaw in an 
otherwise homogeneous specimen depends upon the differential in film den- 
sity (i. e. , contrast) which it produces. The geometrical size of the flaw 
(as projected on the radiograph) is also important since low contrast images 
of small size are more difficult to discern by visual inspection than larger 
images of comparable contrast. The most important parameters influencing 
the detectability of the image of the flaw on the final radiograph are: 

(1) Beam quality, which in turn depends upon 

(a) The degree of collimation of the incident radiation 

(b) The absence of multiple scattering (diffusion) of 
radiation in the specimen 

(2) Exposure, which depends upon the product of 

(a) Radiation source strength 

(b) Duration of exposure (sec. ) 

(3) Effective source size (which contributes to geometrical 
unsharpness of the image) 

(4) Film-to-focus distance 

(5) Film-to-specimen distance 

(6) Film characteristic curve and speed 

(7) Presence and type of image intensifying screens (lead foil 
or fluourescent powder) 

(8) Physical stability of source, specimens, and film cassette 
during exposure 

(9) Uniformity of film development 

(10) Visual acuity and expertness of film interpreter. 



60 



In the usual industrial or field environment, a radiographic sensitivity of 
2 percent is ordinarily considered good. By convention, "2-percent sensi- 
tivity" means that, if a penetrameter (made of the same material as the 
specimen) whose thickness (T) is 2 percent that of the specimen and which 
contains a circular hole whose diameter is twice the thickness of the pene- 
trameter (2T), is placed in contact with specimen (on the source side) and 
the assembly is radiographed for a fixed set of conditions, the image of the 
2T-hole in the penetrameter can be unambiguously discerned by a qualified 
radiograph reader. Under carefully optimized conditions, 0.7-percent 
radiography can be achieved; this means the image of a lT-hole in a pene- 
trameter, the thickness of which is 1 percent of the specimen thickness, 
can be discernedw ' ). 

In terms of applicability to the inspection of bridges which are in 
unrestricted service, the following general observations may be made: 

(1) Normal vibration would require the use of fast-pulse X-ray 
sources. These are available but may have inadequate total 
exposure for the thicknesses of concrete required. 

(2) Thicknesses of concrete of interest would probably require 
X-ray units capable of 600 KV and upward; 

(3) A clearance of 2 to 3 ft between the X-ray source and the 
area to be radiographed would be necessary; 

(4) Skills required are high; 

(5) The health hazard associated with radiation requires a 
considerable safety program. 

Currently available nonfilm radiographic methods (X-ray, T. V. , etc.) have 
resolution inferior to film. Digital image enhancement, though greatly 
increasing crack detectability, is not yet economic for large-scale application. 

While some work on the use of gamma radiography in the inspection 
of reinforced concrete has been reported,' *' personal communications have 
indicated the cost to be prohibitive for bridge inspection and the expected 
resolution to be poor. The decision to use or not use radiography should be 
based mainly on there being no viable alternative method. 

STRAIN GAGE 

The vibrating wire principle for the measurement of strain in concrete 
provides a means of making long term studies of dams or pressure vessels 
since they exhibit good stability over a number of years'- 5 "'. It has been 

61 



reported that long term measurements of strain could be made to better 
than 1 x 10~". The principle of the vibrating wire gage relies on the 
mesurement of the natural frequency of a wire under tension. Any change 
of tension causes a corresponding change of frequency. By considering 
the gage length, properties of the wire and the design of the gage, a rela- 
tionship between strain and frequency can be expressed. 

The gage generally consists of the wire, end clamps, a spacer tube 
enclosing the wire, and an electromagnet for plucking and measuring the 
signal from the wire. A gage length of at least four times the largest 
aggregate size is necessary to avoid excessive errors. The design of the 
gage should be such that the strain field around it is not upset by the pre- 
sence of the gage. Ideally, the stiffness of the gage should match that of 
the concrete it replaces. By the choice of a suitable material for the tube, 
its diameter and thickness, a particular stiffness can be obtained. It has 
been demonstrated that the gage factor, for vibrating wire gages, is affected 
when cast in concrete. Accordingly, the practice has been adopted to 
encapsulate all gages in concrete before use to offer mechanical protection 
and to calibrate a sample of the production lot. The gage factors thus ob- 
tained are then used for all gages in the production lot. If the coefficient of 
expansion for the gage is different from that of the concrete in which it is 
cast, a temperature correction will be required. 

The frequency of the vibrating wire may be measured by balancing 
the signal with a second signal of known frequency or by using a standard 
frequency meter. In most cases, to improve the accuracy, the frequency 
is obtained by timing a known number of cycles-- usually 100. 

In the pressure vessel applications, several hundred vibrating wire 
strain gages have been used(^9). While such an array of strain gages in a 
bridge might very well be used to monitor the serviceability of the bridge, 
it is difficult to see how this method could be retrofitted to existing bridges. 

THERMAL 

Thermal methods, in general, as applied to the detection of defects 
fundamentally consist of establishing a heat-flow or thermal gradient in 
the region of the material being examined and then measuring the local tem- 
perature disturbances caused by the presence of a flaw. Implementation of 
the method involves two general areas: (1) establishing a heat flow in the 
part and (2) detecting local temperature differences. 

Inspection of reinforced concrete would necessitate the use of an 
active test system in which the component under test must be raised or 
lowered in temperature with respect to its surroundings to establish a heat 
flow in the region under inspection. To establish a favorable heat flow 
pattern, it is often necessary to bring the part into thermal contact with an 
appropriate arrangement of thermal sources, heat sinks, conductors, and 
insulators. 

62 



The second area to be considered in thermal testing is the sensing 
or measuring of the local temperature differences which indicate the pre- 
sence of a flaw. Both contact and non-contact methods are available for 
such temperature sensing. In recent years, the greatest emphasis has 
been on the use of non-contacting methods (predominantly infrared radiation 
techniques) for temperature measurement because contact methods, in 
themselves, alter the heat flow at the part surface under test, and they do 
not readily lend themselves to automation. To obtain good sensitivity with 
the infrared method, however, a uniform surface emissivity is required, 
and it is, therefore, extremely important to maintain the test surface 
uniformly clean and free of contaminating liquids or solids. 

In the case of reinforced concrete inspection, the thermal diffusivity 
of the concrete cover and inconsistencies in the thermal conductivity of the 
bond between reinforcing cables and the concrete would effectively destroy 
the sensitivity and resolution of thermal techniques. For a post-tensioned 
beam the presence of a duct (especially metal) around the reinforcing rod 
would cause further diffusion of the thermal energy, thus diminishing the 
sensitivity even further. Even in the case of clean exposed metal surfaces, 
the sensitivity of thermal methods has been limited to the detection of larger 
flaws'-*"'. Thermal techniques have generally been found to have relatively 
poor resolution when compared with other techniques even when applied on 
specimens having thermally "good" surfaces. The technique generally 
involves the use of complex and expensive instrumentation. 

ULTRASONICS 

Ultrasonics consists of using high-frequency, mechanical vibrations, 
introduced into a part by means of a transducer, to observe the interaction 
of this acoustical energy with discontinuities in the part material. A number 
of different techniques such as through-transmission, resonance, pulse-echo, 
etc. , are used for specific applications, with the pulse -echo technique being 
the most widely used. In ultrasonics, an acoustic pulse is launched into the 
sample and propagates as a wave, which, depending on the method of excitation, 
may vibrate the material in various modes. Modes commonly used are longi- 
tudinal (compressional), shear (transverse), plate (Lamb) and surface (Ray- 
leigh ). The pulse travels through the material at its mode's characteristic 
speed of sound; upon encountering a reflecting interface or discontinuity, the 
pulse is reflected and eventually travels back to the initiating transducer or 
another appropriately placed transducer. The distance from the transducer 
to the flaw can be estimated by using the transit time of the acoustic energy 
and the characteristic speed of sound, and the received pulse or signal ampli- 
tude provides a qualitative indication of the relative size of the reflecting area. 



63 



Propagation of ultrasonic energy through any given material is a 
function of the grain size of the material and the wavelength of the acoustic 
energy. Frequencies on the order of several megacycles are commonly 
used in steel, but these frequencies will not propagate any appreciable dis- 
tance through concrete. Ultrasonic inspection of concrete, therefore, is 
performed at much lower frequencies, in the range of a few tens of kilo- 
cycle s(53). These frequencies will propagate adequately through concrete 
(and steel), but their wavelengths are so long in steel that the inspection 
resolution obtained is poor. 

Ultrasonic pulse velocity measurements in concrete have been used 
to assess its quality, w-3, 54, 55) but the experiments have shown that it is 
virtually impossible to detect even the presence of the steel reinforcing 
members, (->&) much less extract any information regarding the condition of 
the steel. 

On the other hand, there is a possibility that ultrasonic scattering 
may be used to estimate the degree of corrosion present on reinforcing 
members, provided access to both sides of the concrete is possible. The 
method essentially would treat the steel reinforcing rod as a cylindrical lens 
whose focal length is a function of the diameter of the rod and the ratio of 
the acoustic velocities of the steel and the concrete. An ultrasonic wave 
passing across the rod has some of its energy deflected off axis by the lens 
effect. The presence of corrosion products, probably possessing an acoustic 
velocity differing from that of either steel or concrete, would act to change 
the focal length of the lens. These effects could be monitored by measurement 
of the ultrasonic energy distributions on the side of the concrete member 
opposite the transmitting transducer. The presence of more than one steel 
reinforcing strand would, of course, complicate the measurements. 



64 



APPENDIX B 

(Worksheets have been omitted for brevity - see Interim 
Report for Method Rating Worksheets) 



65 



APPENDIX C 
REFERENCES 

1. Dunegan, H„ L. , Harris, D O. , and Tatro, C.A., "Fracture Analysis 
by Use of Acoustic Emission, " Engineering Fracture Mechanics , I, 

p. 105, 1968. 

2. Dunegan, H. L. , and Harris, D. O. , "Acoustic Emission-a New 
Nondestructive Testing Tool," Ultrasonics , 7, p. 160, 1969. 

I 

3. Harris, D. O. , Dunegan, H.L., and Tetelman, A. S„ , "Prediction of 
Fatigue Lifetime by Combined Fracture Mechanics and Acoustic 
Emission Techniques," Dunegan Research Corp. Technical Bulletin 
DRC-105. 

4. Fisher, R.M„, and L ally, J„ S. , "Microplasticity Detected by an 
Acoustic Technique", Canadian Journal Physics , 45, p. 1147, 1967. 

5. Frederick, J.R , "Use of Acoustic Emission in Nondestructive 
Testing", U.S. Air Force, Contract No. F3361 5-68 -C-1703 ARPA 
Order No. 1244, May 1969. 

6. Liptai, R. G. , Dunegan, H.L., and Tatro, C.A., "Acoustic Emissions 
Generated During Phase Transformations in Metals and Alloys", 
International Journal Nondestructive Testing , 1, p. 213, 1969. 

7. Kerawala, J. N. , "An Investigation of the Behavior of the Acoustic 
Emission from Commercial Ferrous Materials", Phd. D. Thesis, 
University of Michigan, 1965. 

8. Mason, W. P„ , McSkimin, J. H, , and Shockely, W. , "Ultrasonic 
Observation of Twinning in Tin", Physics Review s, 73, No. 10, 1948. 

9. Schofield, B.H., Bareiss, R.A., and Kyrala, A. A., " WAD C Technical 
Report 58-194", Astia Document No. AD 1555674, 1958. 

10. Green, A. T. , Steele, R.K., and Lockman, C. S. , "Acoustic Veri- 
fication of Structural Integrity of Polaris Chambers", Society, Plastics 
Engineers, Atlantic City, N. J. , January 1964. 

11. Frederick, J. R„, "Acoustic Emission as a Technique for Nondestruc- 
tive Testing", Materials Evaluation , p. 43, February 1970. 

12. Agarwal, A. B. L. , Frederick, J.R., and Felbeck, D„ K. , "Detection 

of Plastic Microstrain in Aluminum by Acoustic Emission", Metallurgical 
Transactions, 1, p. 1069, I960. 



67 



13. Crimmins, P.P., "Correlation of Stress-Wave Emission Charac- 
teristics with Fracture in Aluminum Alloys", George C. Marshall 
Flight Center, NASA Contract NAS 8-21405, June 1969. 

14. Harris, D. O. , "Acoustic Emission Monitoring of Lift Span Cables 
on Dumbarton Bridge", Dept. of Public Works, Division of Bay- 
Toll Crossings, State of California, Dec. 197 2. 

15. Bickle, L. W. , and Smiel, A. J. , "Applicability of Acoustic -Emission 
Techniques to Civil Engineering Research", New Mexico Univ. , 
Albuquerque, Eric H. Wang Civil Engineering Research Facility 

Air Force Weapons Lab., Kirtland AFB, New Mexico, 102p. , June 1975. 

16. Harris, D„ O. , and Dunegan, H. L. , "Acoustic Emission Testing of 
Wire Rope", Materials Evaluation, pp. 1-6, January 1974. 

17. Smith, G. H. , and McMaster, R„ C. , "Inspection and Tracking of 
Welds Using the New Magnetic Reaction Analyzer", Proceedings of 
the 5th International Conference on Nondestructive Testing , The 
Queen's Printer, Ottawa, pp. 108-113, 1969. 

18. Pasley, R. L. , and Birdwell, J. A D , "Eddy Current Testing", 
Nondestructive Testing-A Survey , Chapter 5, NASA-SP-51 13, 
C. Gerald Gardner, Technical Editor, 1973. 

19. Anon. , Welding Inspection, American Welding Society, New York, 
p. 223, 1968. 

20. Forster, F., Proceedings of 5th Conference on Nondestructive 
Testing, The Queen's Printer, Ottawa, p. 209, 1969. 

21. Hochschild, R„ , Electromagnetic Methods of Testing Metals, 
Progress in Nondestructive Testing, Vol. 1, The MacMillan 
Company, New York, pp. 59-109, 1959. 

22. McMaster, R, C. , Nondestructive Testing Handbook, The Ronald 
Press Company, New York, 1963. 

23. Anon. , "R" Meter - Technical Information, James Electronics, 
Inc. 

24. Moore, K. R. , "Rapid Measurement of Concrete Cover on Bridge 
Decks", Public Roads, Vol. 39, No. 2, Sept. 1975. 



68 



25. Moore, D„ G. , Klodt, D„ T. , and Hensen, R. J„ , "Protection of Steel 
in Prestressed Concrete Bridges", Research sponsored by American 
Assoc, of State Highway Officials in cooperation with the Bureau 

of Public Roads; Highway Research Board, Div. of Engineering, 
National Research Council, National Academy of Sciences -National 
Academy of Engineering, 1970. 

26. Moore, R.W., "Electrical Resistivity Instruments for Measuring 
Thickness and Other Characteristics of Pavement Layers", Federal 
Highway Administration, Washington, D. C. Materials Div. , 

p. 59, August 197 2. Prepared by Soils and Exploratory Techniques 
Group 

27. Gould, R.W., Hummel, R. E. , and Lewis, R.O., "Electrical 
Resistance as a Measure of Reinforcing Bar Continuity Sunshine 
Skyway Bridge", Progress Report 1, Dept. of Materials Science 
and Engineering, University of Florida, Gainesville, Florida 3 2611, 
June 29, 1973. 

28. Gould, R.W., "Detection and Prevention of Re-bar Failure in 
Concrete Structure", Final Report, College of Engineering Materials 
Science and- Engineering, University of Florida, Gainesville, Florida 
32611, January 30, 1975. 

29. Leith, E. N. , and Upatnicks, J., "Reconstructed Wavefronts and 
Communication Theory", Journal of the Optical Society of America , 
Vol. 52, No. 10, 1962. 

30. Harris, W. J. , and Clauss, F. J. , "Inspecting Bonded Structures 

by Laser Holography", Metal Progress , Vol. 100, No. 2, August 197L 

31. Alwang, W. G. , Burr, R. and Cavanaugh, L„ A. , "Holographic 
Measurement of Compressor Blade, Turbine Blade and Airframe 
Panel Vibration Distribution", Society of Automotive Engineers, 
International Automotive Engineering Congress, Detroit, Michigan, 
January 1969. 

32. Leith, E. N. , and Vest, CM., "Investigation of Holographic Testing 
Techniques,", Contract No. DAAG46-69-C-0017, April, 1970. 

33. Marom, E. , and Mueller, R. K. , "Nondestructive Early Fatigue 
Detection", Proceedings of 6th Symposium on Nondestructive 
Evaluation of Components and Materials, 1967. 



69 



34. Marom, E. , "Electro-optical Noncontacting Techniques for Sonic 
Fatigue Tests", AFFDL-RE-69-95, December 1969. 

35. Friesem, A.A., and Vest, CM., "Detection of Microfractures by- 
Holographic Interferometry", Applied Optics , Vol. 8, No. 6, June 1969 

36. Wertheim, G. K. , Mo ssbauer Effect: Principles and Applications, 
Academic Press, New York, 1964. 

37. Anon. , "Making a Radiograph", Radiography , Vol. IV, NASA 
CR-61215 (N68-28787). 

38. Kubiak, E„ J. , Johnson, B.A. , and Taylor, R. C. , "Dynamic Infrared 
Detection of Fatigue Cracks (Discussion)", Proceedings 5th Inter- 
national Conference on Nondestructive Testing , 1967. 

39. Hornby, I.W. and Notltingk, B, E. , "Application of the Vibrating - 
Wire Principle for the Measurement of Strain in Concrete", Cent. 
Electr. Res. Lab. , Leatherhead, Surrey, England, Experimental 
Mechanics , Vol. 14, No. 3, pp. 123-128, March 197^ 

40. Anon. , "Holographic Detection of Cracks in Concrete", A. Luxmoore, 
Univ. Coll, Swansea, Glamorgan, Wales; Non-Destructive Testing 
(London), Vol. 6, No. 5, pp. 258-263, Oct. 1973. 

41. Tassios, T. and Oeconomou, C« , "Contribution to the Gamma 
Radiography of Reinforced Concrete Structures", National Tech 
Univ. , Athens, Greece, Material Constr. , Material Struct. , Vol. 4, 
No. 20, pp. 101-106, Mar-Apr 1971. 

42. Kliethermes, J. C, "Repair of Spalling Bridge Decks", Highway 
Research Record, No. 400, pp. 83-92, 1972. 

43. Stratfull, R. F„ , "Half Cell Potentials and the Corrosion of Steel 
in Concrete", California State Div. of Highways, Materials and 
Research Dept. , 32p. Report No. : CA-HY-MR-51 16-7 -72-42, 
M/R-6351 16-7, Prepared in Cooperation with Federal Highway 
Administration, Washington, D. C. , November 1972. 

44. Clear, K. C. and Hay, R.E., "Time -to -Corrosion of Reinforcing 
Steel in Concrete Slabs", Vol. 1 Effect of Mix Design and Construction 
Parameters, Federal Highway Admin. , Washington, D. C. 105p. 
April 1973. 



70 



45. Lewis, D. A. , and Copenhagen, W. J. , "The Corrosion of Reinforcing 
Steel in Concrete in Marine Atmospheres", The South African 
Industrial Chemist, Oct. 1957. 

46. Rehm, G. , "Corrosion of Prestressing Steel," General Report 
submitted at FIP Symposium on Steel for Prestressing, Madrid 
1968. 

47. Tremper, B. , Beaton, J. L., and Stratfull, R. F. , "Corrosion of 
Reinforcing Steel and Repair of Concrete in a Marine Environment", 
Highway Research Board Bulletin 182, 1957. 

48. Spellman, D. L. and Stratfull, R. F. , "Laboratory Corrosion Test 
of Steel in Concrete", California State Div. of Highways, Materials 
and Research Dept. , Report No. : M/R-6351 16-3, 44p. , Sept. 1968. 

49. Roshore, E. C. , "Durability and Behavior of Prestressed Concrete 
Beams", Technical Report No. 6-570, Report 3, Laboratory Tests 
of Weathered Pretensioned Beams. , Army Engineer Waterways 
Experiment Station, Vicksburg, Miss., Oct. 1976. 

50. Snape, E. , "Roles of Composition and Microstructure in Sulfide 
Cracking of Steel", NACE Conference, Cleveland, Ohio, 1968. 

51. Spellman, D. L. and Stratfull, R. F. , "Concrete Variables and 
Corrosion Testing", California State Div. of Highways, Materials 
and Research Dept. , Report No. : M/R-HRB-6351 16-6, 50p. 
Prepared in cooperation with Federal Highway Admin. , Washington, 
D. C. , January 1972. 

52. Spellman, D. L. , and Stratfull, R.F., "Chlorides and Bridge Deck 
Deterioration", Highway Research Record No. 328, 1970. 

53. Bellini, P. X. , "Sonic Testing of Reinforced Concrete", Ohio Dept. 
of Transportation, Columbus, Youngstown State Univ. , Ohio, Dept. 
of Civil Engineering, Federal Highway Admin. , Washington, D. C. , 
Report No. : OHIO-DOT-01 -74, 130p. , December 1973. 

54. Anon. , "Nondestructive Testing of Concrete", No. 378, 7 reports, 
Highway Research Board, Div. of Engineering, National Research 
Council, National Academy of Sciences - National Academy of 
Engineering, Washington, D. C. , 1972. 



71 



55. Moore, W„M„, "Detection of Bridge Deck Deterioration", Summary- 
Report of Research Report No. 130-9, Study 2-18-68-130, Dec. 1972. 

56. Scholar, C. F. , "Performance of Ultrasonic Equipment for Pavement 
Thickness Measurement and Other Highway Applications", Final 
Report, July 1970. 

57. Barton, J. R. , and Kusenberger, F„ N. , "Magnetic Perturbation 
Inspection to Improve Reliability of High Strength Steel Components", 
ASME Design Engineering Conference 69-DE-58, New York, May 1969. 

58. Barton, J.R., and Kusenberger, F. N„ , "Fatigue Damage Detection", 
Metal Fatigue Damage, Mechanism, Detection, Avoidance and Repair , 
STP495, pp. 193-201; 210-212, American Society for Testing and 
Materials, 1971. 

59. Kusenberger, F.N., "Low Cycle Fatigue Damage Detection in Model 
Pressure Vessel", Proceedings of 5th Annual Symposium on Nondes- 
tructive Evaluation of Aerospace and Weapons Systems Components 
and Materials , 1965. 

60. Kusenberger, F,N., and Barton, J. R. , "Development of a Prototype 
Equipment for the Automatic Detection of Fatigue Damage in Helicopter 
Transmission Gears", Proceedings of 7th Symposium on Nondestructive 
Evaluation of Components and Materials in Aerospace, Weapons 
Systems and Nuclear Applications , April 1969. 

61. Kusenberger, F.N. , Lankford, J. , Jr., Francis, P. H„ , and Barton, 
J.R., "Nondestructive Evaluation of Metal Fatigue," AFOSR Scientific 
Report AFOSR-TR-71-1965, April 1971. 

62. Brunarski, L. and Karminski, A. , "The Magnetic Method for Measuring 
the Diameter and the Depth of Reinforcement Below the Surface of 
Concrete", published in Proc. 7th Int. Conf. on NDT; Warsaw, Poland, 
Polish Soc. Mech. Eng. ; Vol. 1, D-28, 4 pp. June 1973. 

63. Saucier, K„ L. , "Evaluation of an Instrument to Detect Presence, 
Size and Depth of Steel Embedded in Portland -Cement Concrete", 
Army Engineer Waterways Experiment Station, Vicksburg, Miss, 
Report for 1964-1965. 



72 



APPENDIX II 

FCP RESEARCH REVIEW CONFERENCES, PROGRAM 
REVIEWS, AND OTHER PRESENTATIONS 



FCP Research Review Conference Presentations 



University Park, Pennsylvania 
Atlanta, Georgia 



23 September 1976 
4 October 1977 



Program Technical Review (FHWA, Washington Offices) 

13-14 December 1976 
Participants: 



Craig Ballinger 
Charles Galambos 
Stan Gordon 
Charles McGogney 
Emile Paulette 
Walter Podolney 
John Barton 
Felix Kusenberger 
Anthony Leone 



FHWA 

FHWA 

FHWA 

FHWA 

FHWA 

FHWA 

SwRI 

SwRI 

Consultant (SwRI) 



Program Technical Review (FHWA, Washington Offices) 

18 March 1977 
Participants: 



Charles Galambos 
Charles McGogney 
Jerar Nishanian 
Robert Varney 
John Barton 



FHWA 
FHWA 
FHWA 
FHWA 
SwRI 



Program Oral Review (Southwest Research Institute) 

18-19 April 1977 
Participants: 



Charles McGogney 
John Barton 
Felix Kusenberger 
Anthony Leone 



FHWA 

SwRI 

SwRI 

Consultant (SwRI) 



II- 1 



Program Design Review (Southwest Research Institute) 

31 Aug. -1 Sept. 1977 
Pa rti cipants: 



Charles McGogney 
John Barton 
Felix Kusenberger 
Armando DeLos Santos 
Ruell Solberg 
Anthony Leone 



FHWA 

SwRI 

SwRI 

SwRI 

SwRI 

Consultant (SwRI) 



Field Site Conference (Salt Lake City, Utah) 

11 November 1977 
Participants; 



c. 


McGogney 


FHWA 


R. 


Sharp 


FHWA 


R. 


Frost 


FHWA 


M. 


Godfrey 


FHWA 


R. 


Behling 


Utah DOT 


A. 


Bloomfield 


Utah DOT 


D. 


Christensen 


Utah DOT 


P. 


Panos 


Utah DOT 


A. 


Peikes 


Utah DOT 


F. 


Bennet 


Utah DOT 


F. 


Kusenberger 


SwRI 


G. 


Ferguson 


SwRI 


A. 


Leone 


Consultant (SwRI) 



Presentation (Southwest Research Institute) 
11 January 1978 



Participants: 

N. Clary 
G. Bolyard 
H. Taylor 
H. Gale 
W. Lindsay 
G. Love 
J. Viner 
J. Ahlskog 
J. Wentworth 
J. Hatton 



FHWA 
FHWA 
FHWA 
FHWA 
FHWA 
FHWA 
FHWA 
FHWA 
FHWA 
FHWA 



II- 2 



APPENDIX III 
PERSONNEL CONTRIBUTING TO OVERALL PROGRAM EFFORT 



The following personnel, listed according to organizational 
affiliation have provided guidance and have technically contributed to 
the subject program. 

Department of Transportation 
Federal Highway Administration 



Washington Headquarters 

C. Ballinger 
C. Galambos 
M. Godfrey 
S. Gordon 
C. Hartbower 

Region 4, Atlanta, Georgia 

R. Hove 



Region 8, Denver, Colorado 

R. Sharp 
Utah Division, Salt Lake City, Utah 

R. Frost 



C. McGogney 
J. Nishanian 
E. Paulette 
W. Podolney 
R. Varney 



State Departments of Transportation 



Florida 



Utah 



H. Burns 

R. Rodriguez 



R. Behling 

F, Bennet 

A. Bloomfield 



J. Roberts 



D. Christensen 
P. Panos 
A. Peikes 



III- 1 



McDonough Brothers Incorporated 

J. McDonough, Jr. 

University of Florida 

M. Self 

Southwest Research Institute 

A. DeLosSantos A. Leone (Consultant) 

G. Ferguson R. Solberg 



III- 2 



APPENDIX IV 

FIELD EVALUATION CONDUCTED AT MANUFACTURED CONCRETE, INC. 

SAN ANTONIO, TEXAS 



A. General 

The field evaluations at Manufactured Concrete, Inc. in San Antonio, 
Texas (formerly McDonough Brothers, Inc.) were undertaken subsequent 
to the field work at Salt Lake City, Utah (see Section II. C. 1 of this report). 
The decision to conduct a second field evaluation was based on the anomalous 
signatures obtained from the post-tensioned girders on the Sixth South 
Street Viaduct at Salt Lake City, and the related problems of interpreting 
magnetic signature records for the detection of fracture and deterioration 
of the prestressing steel elements. The anomalies were later correlated 
with the presence of unanticipated steel elements. Importantly, the 
selection of a second field test site was based on the availability of Texas 
Type "C" girders for inspection since the laboratory work was conducted 
on a 20-ft. (6m) test beam having the Type "C" configuration. This approach 
would permit direct comparison of field inspection results with those 
previously obtained in the laboratory. Fortunately, a large group of 
80-ft. (24m) Type "C" girders were available at Manufactured Concrete, 
Inc. , the fabricators of the 20-ft. (6m) test beam used in the laboratory 
study. Approval to handle and inspect the girders was obtained from the 
State of Texas and the City of San Antonio by FHWA,and an inspection 
schedule was immediately developed which was commensurate with the 
short-term availability of the girders prior to their delivery to the customer 
(City of San Antonio). 

B. Site Description 

Figure IV- 1 shows an overall view of the inspection site and girder 
setup, provided through the excellent support of Manufactured Concrete, Inc. 
Figure IV-2 shows a view of a Type "C" girder being installed 
for inspection. The site layout accommodated the setup of three girders 
simultaneously using available steel reinforced concrete blocks as temporary 
piers to support each end of the 80-ft. (24m) girder approximately 5-ft. (1. 5m) 
above ground level. Girders were transported from the production storage 
location to the test site and set up using the special handling equipment shown 
in Figure IV-2. Subsequent to inspection, each girder was returned to storage. 
Ten (10) 80-ft. (24m) span girders were inspected over approximately 60-ft. 
(18m) of their span. Inspection, including transport of the inspection 
equipment to the test site, initial setup, teardown, etc. was completed in a 
5-day period. Figure IV-3 illustrates views of the inspection equipment 
installed on each of two different girders, one of which was previously 



IV- 1 




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IV- 3 



A. View of Typical Inspection Setup on Production Girder 




B. View of Inspection Setup on Reject Girder 




'. 




FIGURE IV-3. VIEWS OF MAGNETIC INSPECTION EQUIPMENT INSTALLED 
ON GIRDERS AT MANUFACTURED CONCRETE, INC. (formerly 
McDonough Brothers, Inc. , San Antonio, Texas) 



IV- 4 



rejected because of severe honeycomb type defects (see Figure IV-4). As 
will be explained in detail later in this section of the report, the reject 
girder was utilized for special tests in which strand deterioration and 
fracture and separation were simulated in the field. Figure IV -5 presents 
plans showing the structural details of the Texas Type "C" girders 
inspected. A comparison of the details in Figure IV- 5 with those of the 
test beam in Figure 10 of the body of this report shows corresponding 
structural details, except that i) the center two columns of strands for 
the 80-ft. (24m) girders are draped while those of the test beam are not, 
arid ii) the detailed configuration of the holdown fixtures in the two cases 
was different (not shown). 

C. Inspection Procedures and Results 

Typically, inspections were initiated approximately 8-ft. (2.4m) 
from one end of a girder and the first group of scans extended approximately 
34 -ft. (10m) along the length of the girder; this initial scan group was set 
up by using 24-ft. (7.6m) and 15-ft. (4.6m) track lengths placed end-to-end. 
Subsequently, the 25-ft. (7.6m) length of track was moved ahead of the 
15-ft. (4. 6m) track section and again placed end-to-end to facilitate an 
additional 34-ft. (10m) scan group overlapping the first scan group appro- 
ximately 10-ft. (3m). In this manner, <~60-ft. (18m) of each 80-ft. (24m) 
girder was inspected. For each scan group, the inspection head was 
transversely positioned under each strand so that a total of 10 transverse 
locations at 2-in. (5cm) intervals was scanned. On records to be discussed, 
T = 2.0 is a scan track located coincident with the outermost strand in the 
flange; T = 4. is the adjacent strand, T = 6. the next adjacent, etc. 

Figure IV-6 shows typical magnetic inspection records over 
approximately a 25-ft. (7. 6m) length of scan on a production girder for 
four adjacent transverse scan tracks. Note the relatively smooth trace 
(absence of stirrups signals) in Record A at the top of Figure IV-6 and 
the increasing amplitude of the stirrup signals in Records B, C, and D. 
An examination of the structural details in Figure IV- 5 indicates that the 
stirrups (R-bars) end beneath the lower row of pretensioned strand be- 
tween the T = 4.0 and T = 6. (Records B and C) transverse positions; 
correspondingly, the signals from the stirrups (R-bars) are significantly 
more pronounced in Records C and D than they are for Records A 
(essentially no stirrup signature) and B (Figure IV-6). The plans specify 
a stirrup spacing of 1 -ft. (30cm) and the records in Figure IV-6 indicate, 
typically, a 1 -f t. (30cm) spacing although there is evidence of unequal 
spacing (see Record C). The records in Figure IV-6 indicate that the 
spacing between the bottom of the R-bar stirrup and the lower surface of 
the girder, i. e. , coverage over the R-bar, is uniform because of the 
uniform amplitude of the stirrup signal. 

IV-5 





FIGURE IV-4. VIEWS SHOWING "HONEYCOMB" AREAS ON REJECT 
GIRDER [D-3(C-3A), Cast 7-26-77] 



IV-6 




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Figure IV-7 presents selected inspection records near mid-length 
on another production girder showing unequal stirrup spacing in this 
region; this type of stirrup spacing pattern was noted on approximately 
30% of the production girders inspected. Since the plans specify the 
quantity of R-bars (see Figure IV-5) it is anticipated that the closer R-bar 
spacing near mid-span on the girder represents the reduction in spacing 
necessary to achieve the total quantity specified; this condition undoubtedly 
results because of the accumulation of tolerances on the nominal 1-ft. (30cm) 
spacing of the R-bars during steel "tieup". 

For purposes of comparison, Figure IV- 8 illustrates typical 
magnetic inspection records from the laboratory test beam at transverse 
positions of 6. 5, 8.5, and 10. 5-in. (16, 22, 27-cm, respectively) from 
one face of that beam. A direct comparison of records from essentially 
the same transverse positions in the laboratory and the field (Figures IV-7 
and IV-8) show good correspondence in the signature response from stirrups. 
The amplitude of the stirrup signatures in the case of the laboratory test 
beam (see Figure IV-8) appears to be slightly greater than that for the 
production girders; this is anticipated since the bottom of the R-bars for 
the test beam are slightly closer to the lower surface of the beam (less 
concrete coverage) because the pretensioned strand was housed in 9/16-in. 
(1.4cm) I.D. PVC tubing and the R-bars located against the O. D. of the 
PVC tubing (on production girders the R-bars are located against the bottom 
of the pretensioned strand). Again referring to Figures IV-7 and IV-8, 
specifically, Record C in each case, it is evident that the signature from 
the "holdown" for the production girder is lower in amplitude and different 
in shape to that obtained from the test beam. Investigation established 
that the physical configuration of the holdown in the test beam was different 
from that used in the production girders such that significantly different 
magnetic signatures would be anticipated. 

As is evident from the foregoing discussions, there is excellent 
agreement between the results obtained in the laboratory from the 20-ft. 
(6m) section of Type "C" test beam and those obtained in the field from 
the 80-ft. (24m) production girders. In the case of the field girders, 
however, a few anomalous signatures were obtained at random locations 
on some girders which could not be correlated with any structural features 
on the manufacturing drawings. A typical example of such anomalous 
signatures is shown in Record B of Figure IV-9. Such anomalous signa- 
tures were also noted on the reject girder (girder containing the honey- 
comb defect) and the signature locations were physically marked on the 
girder. These locations were subsequently excavated using a pneumatic 
hammer - short pieces of 16-ga. soft iron wire were noted in each of the 
two locations excavated. Such wire is used to "tie" the transverse steel 

IV-9 




.Sitll.'j 




* T = Transverse position of scan from girder face, in inches; 
1 in. = 2. 54cm 



FIGURE IV- 7. MAGNETIC INSPECTION RECORDS FROM 80-FOOT (24m) 
TYPE "C" GIRDER SHOWING "STIRRUP" AND "HOLDOWN" 
SIGNATURES NEAR CENTER OF GIRDER (C-2C-2A, 
Cast 12-29-77) 



IV-10 



See Notes 



{j2!l::::l:5::|::!:|::. 

^f old own: 



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[T.= 8. 5-jj 



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Holdown-M 




Notes: x Flawed strand - 6 of 7 wires removed over 2 in. length 

• Unflawed strand 

* T = Transverse position of scan from girder face in 

inches; 1 in. =2. 54cm 



FIGURE IV-8. TYPICAL MAGNETIC INSPECTION RECORDS IN THE LABORA- 
TORY FROM TYPE "C" TEST BEAM, 20-FOOT (6m) SECTION 



IV-11 



A. Signatures from Soft Iron "Tie Wire" (16 ga) Mockup Tests on Girder 




-| |-«— 3. in. 

—~ (7. 6cm) 



7. 5 in.-*-j 
(19cm) - 



«*-l . in. 

(2. 5cm) 



1 — *-3. in. (7. 6cm) 



Longitudinal Axis of Girder 



B. S 



ignature Corresponding to Visual Indication of "Tie Wire" Scrap 

:ll||i;'i:|!liif: 




View of Lower 
Surface of 
Girder 
(C-2C-2A, 
Cast 1-6-78) 




1 in. 



-\ 



2. 5cm 



FIGURE IV- 9. MAGNETIC SIGNATURES FROM SOFT IRON "TIE WIRE" 
SCRAP PIECES CAST IN LOWER SURFACE OF GIRDER 



IV-12 



to the prestressing steel during fabrication, the excess wire is cut off, 
and the soffit is cleaned with a jet of air to remove the scrap. The tie 
wire scraps were located very near the surface but were covered with a 
very thin coating of concrete. In subsequent inspections of other girders, 
visual inspection at locations corresponding to such signatures as those 
shown in Record B of Figure IV-9, disclosed, in some cases, the 
presence of wire scrap; one such case is illustrated by the photograph 
and record at the bottom of Figure IV-9. 

To further confirm signature features from such sources as 
wire scrap, mockup tests were conducted near the honeycomb end of 
the reject girder by attaching known lengths of wire in each of two 
orientations along the bottom surface of the girder; subsequently, an 
inspection scan was made over the path of the specimens. Record A 
at the top of Figure IV-9 illustrates the results obtained from various 
lengths of "tie wire" scrap ranging from 1-in. (2. 5cm) to 7. 5-in. (19cm) 
in each of two orientations with respect to the longitudinal axis of the 
girder. The signature from wire"d" (1-in., 2.5cm, length) correlates 
well with the more dominant signature shown in Record B below in the 
same figure. Note that signatures from samples "a" and "c" indicate 
that the horizontal distance between the downward- and upward-going 
peaks appear to correlate well with the length of the sample. Also note 
that in the case of the sample oriented transverse to the axis of the 
girder (and scan direction) wire "b", a very small signature amplitude 
is obtained compared to that from the same sample length but oriented 
parallel to the axis of the girder (wire "a"). Continuing to examine the 
data in Figure IV-9, Record A, it is pointed out that the signature polarity 
(as viewed from left- to- right) is downward-going and then upward-going 
which is opposite to that obtained from a deterioration type flaw (fracture 
or loss-of-section). This "opposite polarity" type signal is indicative 
of the presence of a piece of ferromagnetic material (iron or steel) with 
no connecting ferromagnetic material in the immediately adjacent region. 
Importantly, the evaluation of these signatures, including the details of 
their features, would be considerably more difficult if they were located 
in a transverse position where larger amplitude stirrup signatures were 
present. 

Field evaluations were also conducted on simulated fracture, and 
loss-of-section in strand using the reject girder. Concrete was excavated 
from the region surrounding a pretensioned strand at several locations; 
subsequently, an acetylene torch was used to either completely cut 
through a strand to simulate a separated fracture or to partially cut a 
strand section to simulate loss-of-section due to corrosion. Photographs 



IV- 13 



of the typical flaws fabricated are shown in Figure IV- 10, and the corre- 
sponding magnetic signatures obtained from scanning these regions are 
shown in Figures IV- 11 and IV- 12. Magnetic records were also obtained 
before flaw fabrication. Figure IV- 11 shows typical results obtained 
from flaws "a", "b", and "c". Results obtained from the flaws fabricated 
in the production type girder containing pretensioned strands are in good 
agreement with the laboratory results obtained from the test beam 
containing untensioned strand. It is pointed out in Record A of Figure IV- 11 
that the horizonal distance between the upward- going and downward- going 
peaks (peak separation) from flaw "a" is greater than that from flaw " c" 
in Record B of the same figure, as was anticipated, since flaw "a n is in 
a second-row strand while flaw " c" is in a first-row strand (see sketch 
at upper left in Figure IV- 11). Note the signatures from the scrap wire 
pointed out in Record B; these signals were present before the flaws were 
fabricated as shown in Record C. It is also pointed out that as the inspec- 
tion head is transversely positioned beneath strands adjacent to the flawed 
strand, the flaw signature becomes correspondingly smaller in amplitude 
as anticipated. Figure IV- 12 shows additional results, from flaws "d" and 
"e", both simulating strand fracture with end separation of approximately 
1-in. (2. 5cm). Again, these results are in agreement with those previously 
obtained in the laboratory. Note the difficulty of recognizing the presence 
of flaw signatures on scan records for the various scan track locations 
(transverse) where the stirrup signature amplitudes are significant (see 
Records A and B in Figure IV-12). 

D. Cursory Flaw Signature Enhancement Investigations 

The one -week evaluation conducted at Manufactured Concrete, Inc. 
on ten unerected girders dramatically demonstrated the inherent capabilities 
of the magnetic method to assess configurational features and the structural 
condition of the prestressing steel in concrete bridge beams. These recent 
evaluations, however, also confirm the need for signature enhancement and 
signature analyses if the full potential of the magnetic method is to be 
realized. Because of continuing interest in our nation's bridge inspection 
problems and confidence in the capabilities of the magnetic method for the 
inspection of prestressed concrete bridge structural members, Southwest 
Research Institute undertook cursory signature enhancement investigations 
with the approval of and at no cost to the Government. These investigations 
and the encouraging results obtained are briefly summarized below. 



IV -14 



A. Flaw a - Tapered Strand Separation 




B. Flaw b - ~50% Strand Section Loss 




2. 5cm 
C. Flaws c, d, e - ^1 in. Strand Separation 




Note: See Figure IV- 11 and IV- 12 for magnetic signatures from flaws 

FIGURE IV- 10. PHOTOGRAPHS OF FLAWS FABRICATED IN PRETENSIONED 
STRANDS OF REJECT GIRDER [D-3(C-3A), Cast 7-26-77] 



IV-15 




'law 


Strand 


Flaw Type* 


a 


#11 


Tapered Strand Separation 


b 


#1 


-*50% Section Loss 


c 


#2 


*-l in. (2. 5cm) Strand 
Separation 




#3 


No flaws 



'#2 Strand 
A. Signatures from Flaws a, b, c for Scan Centered Under Strand #1 




tT = 2. 0**fi 

itawwi 



B. Signatures from Flaws a, b, c for Scan Centered Under Strand #2 




C. Before Flaws a, b, c Fabricated in St rands # 1, #2, #11 




D. Signatures from Flaws a, b, c for Scan Centered Under Strand #3 

— irrnrrrrrnnmrr - " 




Notes: * See Figure IV-10 for photographs of flaws 

** T = Transverse position of scan from face of girder, 
in inches; 1 in. = 2. 54cm 

FIGURE IV -.1.1. MAGNETIC RECORDS SHOWING SIGNATURES FROM FLAWS 
FABRICATED IN STRANDS OF REJECT GIRDER [D-3(C-3A), 
Cast 7-26-77] 



IV-16 




Flaw Strand 
d #9 

e #9 



Flaw Type* 
"'l in. Strand Separation 

<**1 in. Strand Separation 



#2 Strand 



A. Signatures f rom Flaws d, e for Scan Centered Under Strand #8 
'Flaw diri k k *!IM}#H ** W 4|| -Flaw eH U ^ ill! uilMJM ;^Stirrups4 4lU 




C. Signatures from Flaws d, e for Scan Centered Under Strand #9 




D. Signatures from Flaws d, e for Scan Centered Under Strand #10 

TTTTrrrnTTm- 




Notes: * See Figure IV- 10 for photographs of flaws 

** T = Transverse position of scan from face of girder, 
in inches; 1 in. = 2. 54cm 



FIGURE IV- 12. MAGNETIC RECORDS SHOWING SIGNATURES FROM FLAWS 
FABRICATED IN STRANDS OF REJECT GIRDER [D-3(C-3A), 
Cast 7-26-77] 



IV- 17 



The signature enhancement investigations were aimed at evaluating the 
configurational characteristics of plant-produced Type "C" girders; namely, 
the periodic, uniform amplitude which is characteristic of stirrup signatures 
and the low amplitude of stirrup signatures for scans beneath the outermost 
strands (e.g., record A, for T = 2. 0, Figure IV-6) were considered. Tests 
were conducted on the laboratory Texas Type "C " test beam for 1/2 -in. 
(1. 3cm) strand containing manufactured flaws inserted in the T = 2.0 location; 
the results of these tests are shown in Figure IV-13. The top record in 
Figure IV-13 shows the stirrup signatures obtained over an 8-ft. (2.4m) scan 
with a vertical sensitivity ten times greater than that normally used in pre- 
vious laboratory and field test data recording. The gentle downward curvature 
of the baseline from left to right in all records of Figure IV-13 results from 
magnetic "end effects". The second record from the top in Figure IV-13 shows 
the signatures obtained from flaws manufactured in one wire of 7 -wire strand; 
importantly, flaw B, a fractured wire (simulated) with an end separation of 
only 0. 05-in. (^1.25mm), is detectable with prior knowledge of the stirrup 
signatures (compare records A and B in Figure IV-13 for "before" and "after" 
flaw conditions, respectively). The record B of Figure IV-13 shows also that 
a fractured wire with 0.25-in. (6. 3mm) end separation (flaw A) is readily 
recognizable in the presence of the periodic stirrup signatures of lower uniform 
amplitude. Record C in Figure IV-13 illustrates that signatures from more 
subtle flaws which simulate minimal corrosion deterioration are almost un- 
detectable. Record D in Figure IV-13 shows the outstanding signature obtained 
from a more severe simulated flaw at the significantly increased sensitivity 
used in this test. Photographs of flaws A, B, and E are shown in Figure IV- 14. 

Certainly, the flaw signatures in Figure IV-13 illustrate the excellent 
sensitivity of the magnetic method where there is minimal influence from 
structural steel details such as stirrups. Signature enhancement tests were 
also conducted on the higher sensitivity data previously presented in Figure IV-13, 
Such tests consisted of digitizing the signatures from scanning an unflawed 
strand and flawed strands (containing flaws A and B) and, subsequently, sub- 
tracting the two scans point-by-point (hereafter referred to as a "differencing 
process"). Reconstructed analog versions of the digitized scan data before 
and after the "differencing process" are illustrated in Figure IV- 15. The 
lower trace shows that the fracture of a single wire in a 7 -wire strand with 
an end separation of 0. 05-in. (^>1.25mm) is readily detectable and illustrates 
the potential improvements which could be achieved by further reducing the 
influence of stirrup signatures through application of modern data processing 
techniques. 

A modification of the "differencing process" was applied also to field 
data records from the evaluations conducted at Manufactured Concrete, Inc. 
and these results are very encouraging. Strip chart recordings from the reject 



IV-li 




■!-. ,- in. ,^;,. . r -::L_-;..4 ------ Pt J.;J-4~ -0. 2V. 



T~~r~r~' T^j" 0. 5-in. a, 




= A=I: ; :;:|- 

nrirtttnTntr 

T = 2. typ — 



No Flaw 



7 wire strand 



Flaw B__ .;_ Jli..[FlawA: 



i(ft"3E 




Flaw B 

~0. 05-in. gap 

in one wire 



rFlaw A 
0. 25-in. gap 
in one wire 



II* 



3 



W 



\ r-Flaw D 

0. 5-in. long 
reduction one- 
half wire dp. 

— i h- 



Flaw C 

/*>0. 05-in. gap 

thru one-half 

wire 

h 



1 |nr1 _ rT-|T- 



yStmrrTZ^ gjgg 



■""Jl! 



liSiiiHiiP 



■ Flaw E 



0. 25-in. gap in 6 wires 
center wire remaining 




*l h 



Notes: X Flawed Strand, • Unflawed Strand, T - transverse scan reading 
1 in. - 2. 54 cm 

Vertical baseline shift (left to right) result of end effects because 
scan is near left end of beam. 

FIGURE IV- 13. HIGH SENSITIVITY MAGNETIC RECORDS 
ILLUSTRATING INFLUENCE OF STIRRUP 
SIGNATURES ON DETECTION OF SMALL 
FLAWS 



IV- 19 



0. 5 - in. (1. 3cm) diameter, typ. 




Flaw A 

1/4-in. (0. 64cm) Gap 
in 1 Wire 




Flaw B 

Saw Cut, /^0. 05-in. 
(r*>0. 13cm) Gap in 
1 Wire 




Flaw E 

1/4-in. (0. 64cm) Gap 
in 6 Wires 



See Figure IV- 13 for flaw signatures 

FIGURE IV-14. PHOTOGRAPHS OF TYPICAL FLAWS 
FABRICATED IN 7-WIRE STRAND 



IV -20 



VS. 



% Scan A - Unflawed Strand 



v^ 



N ^ 



A 



4 



s 



v 



% 



* \ 

\ Scan B- Flawed Strand 

\ A A 



AA £ v 






Flaw Signature Enhancement : 

(Scan B - Scan A) A ^^J^l/4-in. Gap in 6 Wires 



Saw Cut (/v- 0. 05-in. gap) 



V- 



w in 1 Wire *\ j*"^ • / '" , Wto& 



8 18 28 38 48 58 66 78 86 98 166 118 
SCAN DISTANCE < INCHES) 



FIGURE IV- 15. DIGITIZED HIGH SENSITIVITY RECORDS SHOWING 
SIGNATURE ENHANCEMENT FOR SMALL FLAWS 

IV-21 



girder, after the introduction of flaws, were digitized in the laboratory for 
scans at five different transverse locations. A reproduction of the recon- 
structed digitized signatures are presented in Figure IV- 16; comparison of 
these records for T = 2. 0, 4. and 6. with the original analog records 
previously shown in Figure IV- 11 illustrates the fidelity of the digitized 
signature. A reproduction of the digitized signatures from T = 6. and 
8. is repeated at the top of Figure IV- 17 for reference purposes in the 
following discussion. The modified differencing process consisted of: 

i) adjusting the vertical amplitude of the T = 6. record, 

on a point-by-point basis, such that the average amplitude 
of the stirrup signatures was equal to that for the stirrup 
signatures from the adjacent scan location, T = 8.0; 

ii) subsequently the T = 8. record was subtracted, point- 
by-point, from the normalized T = 6. record; and 

iii) the result was reconstructed in analog form as shown 
at the bottom of Figure IV- 17. 

This bottom record in Figure IV- 17 illustrates a dramatic increase in the 
detectability of a flaw signature; compare the signature pointed out by the 
arrow in the lower record of Figure IV-17 with the corresponding signature 
in the top record of the same figure. Importantly, these signature enhance- 
ment results were obtained on field data without prior knowledge of the 
"before flaw" condition. 

Although the recent signature enhancement investigations described 
above were limited, the results obtained are extremely encouraging and 
illustrate the significant potential improvements in capability of the magnetic 
method to assess deterioration of the prestressing steel in concrete bridge 
structural members. 

E. Summary 

A comparison of the overall signature response was obtained between 
production Type "C " girders in the field using the magnetic inspection equip- 
ment with those obtained from a Type "C" test beam in the laboratory. The 
agreement was excellent. Such signature agreement is noted for the response 
from the overall configuration of the structural steel elements as well as for 
response from simulated flaws (fracture and loss-of-section). Signatures 
obtained on production girders not previously observed from the laboratory 
test beam are the result of either the presence of minor, superfluous, non- 
structural items (wire scrap) or holdowns of a different physical configuration 
which produce a different magnetic response. The results of recent signature 
enhancement investigations are very encouraging and provide a viable approach 
for improving the magnetic detection of flaws in the pretensioning strands of 
Type "C" girders. 

IV-22 



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FEDERALLY COORDINATED PROGRAM (FCP) OF HIGHWAY 
RESEARCH AND DEVELOPMENT 



The Offices of Research and Development (R&D) of 
the Federal Highway Administration (FHWA) are 
responsible for a broad program of staff and contract 
research and development and a Federal-aid 
program, conducted by or through the State highway 
transportation agencies, that includes the Highway 
Planning and Research (HP&R) program and the 
National Cooperative Highway Research Program 
(NCHRP) managed by the Transportation Research 
Board. The FCP is a carefully selected group of proj- 
ects that uses research and development resources to 
obtain timely solutions to urgent national highway 
engineering problems.* 

The diagonal double stripe on the cover of this report 
represents a highway and is color-coded to identify 
the FCP category that the report falls under. A red 
stripe is used for category 1, dark blue for category 2, 
light blue for category 3, brown for category 4, gray 
for category 5, green for categories 6 and 7, and an 
orange stripe identifies category 0. 

FCP Category Descriptions 

1. Improved Highway Design and Operation 
for Safety 

Safety R&D addresses problems associated with 
the responsibilities of the FHWA under the 
Highway Safety Act and includes investigation of 
appropriate design standards, roadside hardware, 
signing, and physical and scientific data for the 
formulation of improved safety regulations. 

2. Reduction of Traffic Congestion, and 
Improved Operational Efficiency 

Traffic R&D is concerned with increasing the 
operational efficiency of existing highways by 
advancing technology, by improving designs for 
existing as well as new facilities, and by balancing 
the demand-capacity relationship through traffic 
management techniques such as bus and carpool 
preferential treatment, motorist information, and 
rerouting of traffic. 

3. Environmental Considerations in Highway 
Design, Location, Construction, and Opera- 
tion 

Environmental R&D is directed toward identify- 
ing and evaluating highway elements that affect 

• The complete seven-volume official statement of the FCP is available from 
the National Technical Information Service, Springfield, Va. 22161. Single 
copies of the introductory volume are available without charge from Program 
Analysis (HRD-3), Offices of Research and Development, Federal Highway 
Administration, Washington, D.C. 20590. 



the quality of the human environment. The goals 
are reduction of adverse highway and traffic 
impacts, and protection and enhancement of the 
environment. 

4. Improved Materials Utilization and 
Durability 

Materials R&D is concerned with expanding the 
knowledge and technology of materials properties, 
using available natural materials, improving struc- 
tural foundation materials, recycling highway 
materials, converting industrial wastes into useful 
highway products, developing extender or 
substitute materials for those in short supply, and 
developing more rapid and reliable testing 
procedures. The goals are lower highway con- 
struction costs and extended maintenance-free 
operation. 

5. Improved Design to Reduce Costs, Extend 
Life Expectancy, and Insure Structural 
Safety 

Structural R&D is concerned with furthering the 
latest technological advances in structural and 
hydraulic designs, fabrication processes, and 
construction techniques to provide safe, efficient 
highways at reasonable costs. 

6. Improved Technology for Highway 
Construction 

This category is concerned with the research, 
development, and implementation of highway 
construction technology to increase productivity, 
reduce energy consumption, conserve dwindling 
resources, and reduce costs while improving the 
quality and methods of construction. 

7. Improved Technology for Highway 
Maintenance 

This category addresses problems in preserving 
the Nation's highways and includes activities in 
physical maintenance, traffic services, manage- 
ment, and equipment. The goal is to maximize 
operational efficiency and safety to the traveling 
public while conserving resources. 

0. Other New Studies 

This category, not included in the seven-volume 
official statement of the FCP, is concerned with 
HP&R and NCHRP studies not specifically related 
to FCP projects. These studies involve R&D 
support of other FHWA program office research. 



DOT LIBRARY 



DDDS71M3